Add WebClient metrics support

This commit adds support for Actuator Metrics for WebClient.
This support mirrors the current behavior for `RestTemplate`, reusing
the same metric name `"http.client.requests"` and tags.

`WebClient` is instrumented by a `MetricsWebClientFilterFunction` which
is applied by a `WebClientCustomizer`. This instrumentation happens
automatically only if you create an instance of `WebClient` using an
auto-configured `WebClient.Builder` bean.

This infrastructure is reusing de facto the `MeterFilter` that has been
added for `RestTemplate` in order to limit the "uri" tag cardinality.

Closes gh-12228

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/client/DefaultWebClientExchangeTagsProvider.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+import java.util.Arrays;
+
+import io.micrometer.core.instrument.Tag;
+
+import org.springframework.web.reactive.function.client.ClientRequest;
+import org.springframework.web.reactive.function.client.ClientResponse;
+
+/**
+ * Default implementation of {@link WebClientExchangeTagsProvider}.
+ *
+ * @author Brian Clozel
+ * @since 2.1.0
+ */
+public class DefaultWebClientExchangeTagsProvider
+		implements WebClientExchangeTagsProvider {
+
+	@Override
+	public Iterable<Tag> tags(ClientRequest request, ClientResponse response,
+			Throwable throwable) {
+		Tag method = WebClientExchangeTags.method(request);
+		Tag uri = WebClientExchangeTags.uri(request);
+		Tag clientName = WebClientExchangeTags.clientName(request);
+		if (response != null) {
+			return Arrays.asList(method, uri, clientName,
+					WebClientExchangeTags.status(response));
+		}
+		else {
+			return Arrays.asList(method, uri, clientName,
+					WebClientExchangeTags.status(throwable));
+		}
+	}
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/client/MetricsWebClientCustomizer.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+import io.micrometer.core.instrument.MeterRegistry;
+
+import org.springframework.boot.web.reactive.function.client.WebClientCustomizer;
+import org.springframework.web.reactive.function.client.WebClient;
+
+/**
+ * {@link WebClientCustomizer} that configures the {@link WebClient}
+ * to record request metrics.
+ *
+ * @author Brian Clozel
+ * @since 2.1.0
+ */
+public class MetricsWebClientCustomizer implements WebClientCustomizer {
+
+	private final MetricsWebClientFilterFunction filterFunction;
+
+	/**
+	 * Create a new {@code MetricsWebClientFilterFunction} that will record
+	 * metrics using the given {@code meterRegistry} with tags provided by the
+	 * given {@code tagProvider}.
+	 * @param meterRegistry the meter registry
+	 * @param tagProvider the tag provider
+	 * @param metricName the name of the recorded metric
+	 */
+	public MetricsWebClientCustomizer(MeterRegistry meterRegistry,
+			WebClientExchangeTagsProvider tagProvider, String metricName) {
+		this.filterFunction = new MetricsWebClientFilterFunction(meterRegistry,
+				tagProvider, metricName);
+	}
+
+	@Override
+	public void customize(WebClient.Builder webClientBuilder) {
+		webClientBuilder.filters(filterFunctions -> {
+			if (!filterFunctions.contains(this.filterFunction)) {
+				filterFunctions.add(0, this.filterFunction);
+			}
+		});
+	}
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/client/MetricsWebClientFilterFunction.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+import java.util.concurrent.TimeUnit;
+
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.Tag;
+import io.micrometer.core.instrument.Timer;
+import reactor.core.publisher.Mono;
+
+import org.springframework.web.reactive.function.client.ClientRequest;
+import org.springframework.web.reactive.function.client.ClientResponse;
+import org.springframework.web.reactive.function.client.ExchangeFilterFunction;
+import org.springframework.web.reactive.function.client.ExchangeFunction;
+
+/**
+ * {@link ExchangeFilterFunction} applied via a
+ * {@link MetricsWebClientCustomizer} to record metrics.
+ *
+ * @author Brian Clozel
+ * @since 2.1.0
+ */
+public class MetricsWebClientFilterFunction implements ExchangeFilterFunction {
+
+	private final MeterRegistry meterRegistry;
+
+	private final WebClientExchangeTagsProvider tagProvider;
+
+	private final String metricName;
+
+	public MetricsWebClientFilterFunction(MeterRegistry meterRegistry,
+			WebClientExchangeTagsProvider tagProvider, String metricName) {
+		this.meterRegistry = meterRegistry;
+		this.tagProvider = tagProvider;
+		this.metricName = metricName;
+	}
+
+	@Override
+	public Mono<ClientResponse> filter(ClientRequest clientRequest,
+			ExchangeFunction exchangeFunction) {
+		long startTime = System.nanoTime();
+		return exchangeFunction.exchange(clientRequest)
+				.doOnSuccessOrError((clientResponse, throwable) -> {
+					Iterable<Tag> tags = this.tagProvider.tags(clientRequest,
+							clientResponse, throwable);
+					Timer.builder(this.metricName)
+							.tags(tags)
+							.description("Timer of WebClient operation")
+							.register(this.meterRegistry)
+							.record(System.nanoTime() - startTime, TimeUnit.NANOSECONDS);
+				});
+	}
+
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/client/WebClientExchangeTags.java

@@ -0,0 +1,107 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+import java.io.IOException;
+
+import io.micrometer.core.instrument.Tag;
+
+import org.springframework.http.client.reactive.ClientHttpRequest;
+import org.springframework.web.reactive.function.client.ClientRequest;
+import org.springframework.web.reactive.function.client.ClientResponse;
+import org.springframework.web.reactive.function.client.WebClient;
+
+/**
+ * Factory methods for creating {@link Tag Tags} related to a request-response exchange
+ * performed by a {@link WebClient}.
+ *
+ * @author Brian Clozel
+ * @since 2.1.0
+ */
+public final class WebClientExchangeTags {
+
+	private static final String URI_TEMPLATE_ATTRIBUTE = WebClient.class.getName() + ".uriTemplate";
+
+	private static final Tag IO_ERROR = Tag.of("status", "IO_ERROR");
+
+	private static final Tag CLIENT_ERROR = Tag.of("status", "CLIENT_ERROR");
+
+	private WebClientExchangeTags() {
+	}
+
+	/**
+	 * Creates a {@code method} {@code Tag} for the
+	 * {@link ClientHttpRequest#getMethod() method} of the given {@code request}.
+	 * @param request the request
+	 * @return the method tag
+	 */
+	public static Tag method(ClientRequest request) {
+		return Tag.of("method", request.method().name());
+	}
+
+	/**
+	 * Creates a {@code uri} {@code Tag} for the URI path of the given {@code request}.
+	 * @param request the request
+	 * @return the uri tag
+	 */
+	public static Tag uri(ClientRequest request) {
+		String uri = (String) request.attribute(URI_TEMPLATE_ATTRIBUTE)
+				.orElseGet(() -> request.url().getPath());
+		return Tag.of("uri", extractPath(uri));
+	}
+
+	private static String extractPath(String url) {
+		String path = url.replaceFirst("^https?://[^/]+/", "");
+		return path.startsWith("/") ? path : "/" + path;
+	}
+
+	/**
+	 * Creates a {@code status} {@code Tag} derived from the
+	 * {@link ClientResponse#statusCode()} of the given {@code response}.
+	 * @param response the response
+	 * @return the status tag
+	 */
+	public static Tag status(ClientResponse response) {
+		return Tag.of("status", response.statusCode().toString());
+	}
+
+	/**
+	 * Creates a {@code status} {@code Tag} derived from the
+	 * exception thrown by the client.
+	 * @param throwable the exception
+	 * @return the status tag
+	 */
+	public static Tag status(Throwable throwable) {
+		return throwable instanceof IOException ? IO_ERROR : CLIENT_ERROR;
+	}
+
+	/**
+	 * Create a {@code clientName} {@code Tag} derived from
+	 * the {@link java.net.URI#getHost host}
+	 * of the {@link ClientRequest#url() URL} of the given {@code request}.
+	 * @param request the request
+	 * @return the clientName tag
+	 */
+	public static Tag clientName(ClientRequest request) {
+		String host = request.url().getHost();
+		if (host == null) {
+			host = "none";
+		}
+		return Tag.of("clientName", host);
+	}
+
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/client/WebClientExchangeTagsProvider.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+import io.micrometer.core.instrument.Tag;
+
+import org.springframework.web.reactive.function.client.ClientRequest;
+import org.springframework.web.reactive.function.client.ClientResponse;
+
+/**
+ * {@link Tag Tags} provider for an exchange performed by a
+ * {@link org.springframework.web.reactive.function.client.WebClient}.
+ *
+ * @author Brian Clozel
+ * @since 2.1.0
+ */
+@FunctionalInterface
+public interface WebClientExchangeTagsProvider {
+
+	/**
+	 * Provide tags to be associated with metrics for the client exchange.
+	 * @param request the client request
+	 * @param response the server response (may be {@code null})
+	 * @param throwable the exception (may be {@code null})
+	 * @return tags to associate with metrics for the request and response exchange
+	 */
+	Iterable<Tag> tags(ClientRequest request, ClientResponse response, Throwable throwable);
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/client/package-info.java

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Actuator support for
+ * {@link org.springframework.web.reactive.function.client.WebClient} metrics.
+ */
+package org.springframework.boot.actuate.metrics.web.reactive.client;

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/server/WebFluxTagsProvider.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,7 +33,7 @@ public interface WebFluxTagsProvider {
 	/**
 	 * Provides tags to be associated with metrics for the given {@code exchange}.
 	 * @param exchange the exchange
-	 * @param ex the current exception (may be {@code null}
+	 * @param ex the current exception (may be {@code null})
 	 * @return tags to associate with metrics for the request and response exchange
 	 */
 	Iterable<Tag> httpRequestTags(ServerWebExchange exchange, Throwable ex);

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/web/reactive/client/DefaultWebClientExchangeTagsProviderTests.java

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+
+import java.io.IOException;
+import java.net.URI;
+
+import io.micrometer.core.instrument.Tag;
+import org.junit.Before;
+import org.junit.Test;
+
+import org.springframework.http.HttpMethod;
+import org.springframework.http.HttpStatus;
+import org.springframework.web.reactive.function.client.ClientRequest;
+import org.springframework.web.reactive.function.client.ClientResponse;
+import org.springframework.web.reactive.function.client.WebClient;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link DefaultWebClientExchangeTagsProvider}
+ *
+ * @author Brian Clozel
+ */
+public class DefaultWebClientExchangeTagsProviderTests {
+
+	private static final String URI_TEMPLATE_ATTRIBUTE = WebClient.class.getName() + ".uriTemplate";
+
+	private WebClientExchangeTagsProvider tagsProvider = new DefaultWebClientExchangeTagsProvider();
+
+	private ClientRequest request;
+
+	private ClientResponse response;
+
+	@Before
+	public void setup() {
+		this.request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.org/projects/spring-boot"))
+				.attribute(URI_TEMPLATE_ATTRIBUTE, "http://example.org/projects/{project}")
+				.build();
+		this.response = mock(ClientResponse.class);
+		given(this.response.statusCode()).willReturn(HttpStatus.OK);
+	}
+
+	@Test
+	public void tagsShouldBePopulated() {
+		Iterable<Tag> tags = this.tagsProvider.tags(this.request, this.response, null);
+		assertThat(tags).containsExactlyInAnyOrder(
+				Tag.of("method", "GET"), Tag.of("uri", "/projects/{project}"),
+				Tag.of("clientName", "example.org"), Tag.of("status", "200"));
+	}
+
+	@Test
+	public void tagsWhenNoUriTemplateShouldProvideUriPath() {
+		ClientRequest request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.org/projects/spring-boot"))
+				.build();
+		Iterable<Tag> tags = this.tagsProvider.tags(request, this.response, null);
+		assertThat(tags).containsExactlyInAnyOrder(
+				Tag.of("method", "GET"), Tag.of("uri", "/projects/spring-boot"),
+				Tag.of("clientName", "example.org"), Tag.of("status", "200"));
+	}
+
+	@Test
+	public void tagsWhenIoExceptionShouldReturnIoErrorStatus() {
+		Iterable<Tag> tags = this.tagsProvider.tags(this.request,
+				null, new IOException());
+		assertThat(tags).containsExactlyInAnyOrder(
+				Tag.of("method", "GET"), Tag.of("uri", "/projects/{project}"),
+				Tag.of("clientName", "example.org"), Tag.of("status", "IO_ERROR"));
+	}
+
+	@Test
+	public void tagsWhenExceptionShouldReturnClientErrorStatus() {
+		Iterable<Tag> tags = this.tagsProvider.tags(this.request,
+				null, new IllegalArgumentException());
+		assertThat(tags).containsExactlyInAnyOrder(
+				Tag.of("method", "GET"), Tag.of("uri", "/projects/{project}"),
+				Tag.of("clientName", "example.org"), Tag.of("status", "CLIENT_ERROR"));
+	}
+
+}

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/web/reactive/client/MetricsWebClientCustomizerTests.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+import io.micrometer.core.instrument.MeterRegistry;
+import org.junit.Before;
+import org.junit.Test;
+
+import org.springframework.web.reactive.function.client.ExchangeFilterFunction;
+import org.springframework.web.reactive.function.client.WebClient;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link MetricsWebClientCustomizer}
+ *
+ * @author Brian Clozel
+ */
+public class MetricsWebClientCustomizerTests {
+
+	private MetricsWebClientCustomizer customizer;
+
+	private WebClient.Builder clientBuilder;
+
+	@Before
+	public void setup() {
+		this.customizer = new MetricsWebClientCustomizer(mock(MeterRegistry.class),
+				mock(WebClientExchangeTagsProvider.class), "test");
+		this.clientBuilder = WebClient.builder();
+	}
+
+	@Test
+	public void customizeShouldAddFilterFunction() {
+		this.clientBuilder.filter(mock(ExchangeFilterFunction.class));
+		this.customizer.customize(this.clientBuilder);
+		this.clientBuilder.filters(filters ->
+				assertThat(filters)
+						.hasSize(2)
+						.first().isInstanceOf(MetricsWebClientFilterFunction.class));
+	}
+
+	@Test
+	public void customizeShouldNotAddDuplicateFilterFunction() {
+		this.customizer.customize(this.clientBuilder);
+		this.clientBuilder.filters(filters -> assertThat(filters).hasSize(1));
+		this.customizer.customize(this.clientBuilder);
+		this.clientBuilder.filters(filters ->
+				assertThat(filters)
+						.hasSize(1)
+						.first().isInstanceOf(MetricsWebClientFilterFunction.class));
+	}
+}

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/web/reactive/client/MetricsWebClientFilterFunctionTests.java

@@ -0,0 +1,119 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+import java.io.IOException;
+import java.net.URI;
+
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.MockClock;
+import io.micrometer.core.instrument.simple.SimpleConfig;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import org.junit.Before;
+import org.junit.Test;
+import reactor.core.publisher.Mono;
+
+import org.springframework.http.HttpMethod;
+import org.springframework.http.HttpStatus;
+import org.springframework.web.reactive.function.client.ClientRequest;
+import org.springframework.web.reactive.function.client.ClientResponse;
+import org.springframework.web.reactive.function.client.ExchangeFunction;
+import org.springframework.web.reactive.function.client.WebClient;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+
+/**
+ * Tests for {@link MetricsWebClientFilterFunction}
+ *
+ * @author Brian Clozel
+ */
+public class MetricsWebClientFilterFunctionTests {
+
+	private static final String URI_TEMPLATE_ATTRIBUTE = WebClient.class.getName() + ".uriTemplate";
+
+	private MeterRegistry registry;
+
+	private MetricsWebClientFilterFunction filterFunction;
+
+	private ClientResponse response;
+
+	private ExchangeFunction exchange;
+
+	@Before
+	public void setup() {
+		this.registry = new SimpleMeterRegistry(SimpleConfig.DEFAULT, new MockClock());
+		this.filterFunction = new MetricsWebClientFilterFunction(this.registry,
+				new DefaultWebClientExchangeTagsProvider(), "http.client.requests");
+		this.response = mock(ClientResponse.class);
+		this.exchange = r -> Mono.just(this.response);
+	}
+
+	@Test
+	public void filterShouldRecordTimer() {
+		ClientRequest request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.com/projects/spring-boot"))
+				.build();
+		given(this.response.statusCode()).willReturn(HttpStatus.OK);
+		this.filterFunction.filter(request, this.exchange).block();
+		assertThat(this.registry.get("http.client.requests")
+				.tags("method", "GET", "uri", "/projects/spring-boot", "status", "200").timer()
+				.count()).isEqualTo(1);
+	}
+
+	@Test
+	public void filterWhenUriTemplatePresentShouldRecordTimer() {
+		ClientRequest request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.com/projects/spring-boot"))
+				.attribute(URI_TEMPLATE_ATTRIBUTE, "/projects/{project}")
+				.build();
+		given(this.response.statusCode()).willReturn(HttpStatus.OK);
+		this.filterFunction.filter(request, this.exchange).block();
+		assertThat(this.registry.get("http.client.requests")
+				.tags("method", "GET", "uri", "/projects/{project}", "status", "200").timer()
+				.count()).isEqualTo(1);
+	}
+
+	@Test
+	public void filterWhenIoExceptionThrownShouldRecordTimer() {
+		ClientRequest request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.com/projects/spring-boot"))
+				.build();
+		ExchangeFunction errorExchange = r -> Mono.error(new IOException());
+		this.filterFunction.filter(request, errorExchange)
+				.onErrorResume(IOException.class, t -> Mono.empty()).block();
+		assertThat(this.registry.get("http.client.requests")
+				.tags("method", "GET", "uri", "/projects/spring-boot", "status", "IO_ERROR").timer()
+				.count()).isEqualTo(1);
+	}
+
+	@Test
+	public void filterWhenExceptionThrownShouldRecordTimer() {
+		ClientRequest request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.com/projects/spring-boot"))
+				.build();
+		ExchangeFunction exchange = r -> Mono.error(new IllegalArgumentException());
+		this.filterFunction.filter(request, exchange)
+				.onErrorResume(IllegalArgumentException.class, t -> Mono.empty()).block();
+		assertThat(this.registry.get("http.client.requests")
+				.tags("method", "GET", "uri", "/projects/spring-boot", "status", "CLIENT_ERROR").timer()
+				.count()).isEqualTo(1);
+	}
+
+}

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/web/reactive/client/WebClientExchangeTagsTests.java

@@ -0,0 +1,114 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.reactive.client;
+
+import java.io.IOException;
+import java.net.URI;
+
+import io.micrometer.core.instrument.Tag;
+import org.junit.Before;
+import org.junit.Test;
+
+import org.springframework.http.HttpMethod;
+import org.springframework.http.HttpStatus;
+import org.springframework.web.reactive.function.client.ClientRequest;
+import org.springframework.web.reactive.function.client.ClientResponse;
+import org.springframework.web.reactive.function.client.WebClient;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link WebClientExchangeTags}
+ *
+ * @author Brian Clozel
+ */
+public class WebClientExchangeTagsTests {
+
+	private static final String URI_TEMPLATE_ATTRIBUTE = WebClient.class.getName() + ".uriTemplate";
+
+	private ClientRequest request;
+
+	private ClientResponse response;
+
+	@Before
+	public void setup() {
+		this.request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.org/projects/spring-boot"))
+				.attribute(URI_TEMPLATE_ATTRIBUTE, "http://example.org/projects/{project}")
+				.build();
+		this.response = mock(ClientResponse.class);
+		given(this.response.statusCode()).willReturn(HttpStatus.OK);
+	}
+
+	@Test
+	public void method() {
+		assertThat(WebClientExchangeTags.method(this.request))
+				.isEqualTo(Tag.of("method", "GET"));
+	}
+
+	@Test
+	public void uriWhenAbsoluteTemplateIsAvailableShouldReturnTemplate() {
+		assertThat(WebClientExchangeTags.uri(this.request))
+				.isEqualTo(Tag.of("uri", "/projects/{project}"));
+	}
+
+	@Test
+	public void uriWhenRelativeTemplateIsAvailableShouldReturnTemplate() {
+		this.request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.org/projects/spring-boot"))
+				.attribute(URI_TEMPLATE_ATTRIBUTE, "/projects/{project}")
+				.build();
+		assertThat(WebClientExchangeTags.uri(this.request))
+				.isEqualTo(Tag.of("uri", "/projects/{project}"));
+	}
+
+	@Test
+	public void uriWhenTemplateIsMissingShouldReturnPath() {
+		this.request = ClientRequest
+				.create(HttpMethod.GET, URI.create("http://example.org/projects/spring-boot"))
+				.build();
+		assertThat(WebClientExchangeTags.uri(this.request))
+				.isEqualTo(Tag.of("uri", "/projects/spring-boot"));
+	}
+
+	@Test
+	public void clientName() {
+		assertThat(WebClientExchangeTags.clientName(this.request))
+				.isEqualTo(Tag.of("clientName", "example.org"));
+	}
+
+	@Test
+	public void status() {
+		assertThat(WebClientExchangeTags.status(this.response))
+				.isEqualTo(Tag.of("status", "200"));
+	}
+
+	@Test
+	public void statusWhenIOException() {
+		assertThat(WebClientExchangeTags.status(new IOException()))
+				.isEqualTo(Tag.of("status", "IO_ERROR"));
+	}
+
+	@Test
+	public void statusWhenClientException() {
+		assertThat(WebClientExchangeTags.status(new IllegalArgumentException()))
+				.isEqualTo(Tag.of("status", "CLIENT_ERROR"));
+	}
+
+}