Support OpenMetrics text format with Prometheus

Update `PrometheusScrapeEndpoint` so that it can produce both classic
Prometheus text output as well as Openmetrics output.

See gh-25564

spring-boot-project/spring-boot-actuator-autoconfigure/src/docs/asciidoc/endpoints/prometheus.adoc

@@ -16,6 +16,15 @@ The resulting response is similar to the following:
 
 include::{snippets}/prometheus/all/http-response.adoc[]
 
+The default response content type is `text/plain;version=0.0.4`.
+The endpoint can also produce `application/openmetrics-text;version=1.0.0` when called with an appropriate `Accept` header, as shown in the following curl-based example:
+
+include::{snippets}/prometheus/openmetrics/curl-request.adoc[]
+
+The resulting response is similar to the following:
+
+include::{snippets}/prometheus/openmetrics/http-response.adoc[]
+
 
 
 [[prometheus-retrieving-query-parameters]]

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/endpoint/web/documentation/PrometheusScrapeEndpointDocumentationTests.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -20,6 +20,7 @@ import io.micrometer.core.instrument.Clock;
 import io.micrometer.core.instrument.binder.jvm.JvmMemoryMetrics;
 import io.micrometer.prometheus.PrometheusMeterRegistry;
 import io.prometheus.client.CollectorRegistry;
+import io.prometheus.client.exporter.common.TextFormat;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.boot.actuate.metrics.export.prometheus.PrometheusScrapeEndpoint;
@@ -31,6 +32,7 @@ import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.docu
 import static org.springframework.restdocs.request.RequestDocumentation.parameterWithName;
 import static org.springframework.restdocs.request.RequestDocumentation.requestParameters;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.header;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
 
 /**
@@ -46,6 +48,14 @@ class PrometheusScrapeEndpointDocumentationTests extends MockMvcEndpointDocument
 		this.mockMvc.perform(get("/actuator/prometheus")).andExpect(status().isOk()).andDo(document("prometheus/all"));
 	}
 
+	@Test
+	void prometheusOpenmetrics() throws Exception {
+		this.mockMvc.perform(get("/actuator/prometheus").accept(TextFormat.CONTENT_TYPE_OPENMETRICS_100))
+				.andExpect(status().isOk())
+				.andExpect(header().string("Content-Type", "application/openmetrics-text;version=1.0.0;charset=utf-8"))
+				.andDo(document("prometheus/openmetrics"));
+	}
+
 	@Test
 	void filteredPrometheus() throws Exception {
 		this.mockMvc

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/export/prometheus/ProducibleTextFormat.java

+/*
+ * Copyright 2012-2021 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
+ *
+ *      https://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.export.prometheus;
+
+import io.prometheus.client.exporter.common.TextFormat;
+
+import org.springframework.boot.actuate.endpoint.http.Producible;
+import org.springframework.util.MimeType;
+import org.springframework.util.MimeTypeUtils;
+
+/**
+ * A {@link Producible} for Prometheus's {@link TextFormat}.
+ *
+ * @author Andy Wilkinson
+ * @since 2.5.0
+ */
+public enum ProducibleTextFormat implements Producible<ProducibleTextFormat> {
+
+	/**
+	 * Openmetrics text version 1.0.0.
+	 */
+	CONTENT_TYPE_OPENMETRICS_100(TextFormat.CONTENT_TYPE_OPENMETRICS_100),
+
+	/**
+	 * Prometheus text version 0.0.4.
+	 */
+	CONTENT_TYPE_004(TextFormat.CONTENT_TYPE_004);
+
+	private final MimeType mimeType;
+
+	ProducibleTextFormat(String mimeType) {
+		this.mimeType = MimeTypeUtils.parseMimeType(mimeType);
+	}
+
+	@Override
+	public MimeType getMimeType() {
+		return this.mimeType;
+	}
+
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/export/prometheus/PrometheusScrapeEndpoint.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -28,8 +28,10 @@ import io.prometheus.client.exporter.common.TextFormat;
 
 import org.springframework.boot.actuate.endpoint.annotation.Endpoint;
 import org.springframework.boot.actuate.endpoint.annotation.ReadOperation;
+import org.springframework.boot.actuate.endpoint.web.WebEndpointResponse;
 import org.springframework.boot.actuate.endpoint.web.annotation.WebEndpoint;
 import org.springframework.lang.Nullable;
+import org.springframework.util.MimeType;
 
 /**
  * {@link Endpoint @Endpoint} that outputs metrics in a format that can be scraped by the
@@ -48,15 +50,25 @@ public class PrometheusScrapeEndpoint {
 		this.collectorRegistry = collectorRegistry;
 	}
 
-	@ReadOperation(produces = TextFormat.CONTENT_TYPE_004)
-	public String scrape(@Nullable Set<String> includedNames) {
+	@ReadOperation(produces = { TextFormat.CONTENT_TYPE_004, TextFormat.CONTENT_TYPE_OPENMETRICS_100 })
+	public WebEndpointResponse<String> scrape(ProducibleTextFormat producibleTextFormat,
+			@Nullable Set<String> includedNames) {
 		try {
 			Writer writer = new StringWriter();
 			Enumeration<MetricFamilySamples> samples = (includedNames != null)
 					? this.collectorRegistry.filteredMetricFamilySamples(includedNames)
 					: this.collectorRegistry.metricFamilySamples();
-			TextFormat.write004(writer, samples);
-			return writer.toString();
+			MimeType contentType = producibleTextFormat.getMimeType();
+			if (producibleTextFormat == ProducibleTextFormat.CONTENT_TYPE_004) {
+				TextFormat.write004(writer, samples);
+			}
+			else if (producibleTextFormat == ProducibleTextFormat.CONTENT_TYPE_OPENMETRICS_100) {
+				TextFormat.writeOpenMetrics100(writer, samples);
+			}
+			else {
+				throw new RuntimeException("Unsupported text format '" + producibleTextFormat.getMimeType() + "'");
+			}
+			return new WebEndpointResponse<>(writer.toString(), contentType);
 		}
 		catch (IOException ex) {
 			// This actually never happens since StringWriter::write() doesn't throw any

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/export/prometheus/PrometheusScrapeEndpointIntegrationTests.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -40,13 +40,21 @@ import static org.assertj.core.api.Assertions.assertThat;
 class PrometheusScrapeEndpointIntegrationTests {
 
 	@WebEndpointTest
-	void scrapeHasContentTypeText004(WebTestClient client) {
+	void scrapeHasContentTypeText004ByDefault(WebTestClient client) {
 		client.get().uri("/actuator/prometheus").exchange().expectStatus().isOk().expectHeader()
 				.contentType(MediaType.parseMediaType(TextFormat.CONTENT_TYPE_004)).expectBody(String.class)
 				.value((body) -> assertThat(body).contains("counter1_total").contains("counter2_total")
 						.contains("counter3_total"));
 	}
 
+	@WebEndpointTest
+	void scrapeCanProduceOpenMetrics100(WebTestClient client) {
+		MediaType openMetrics = MediaType.parseMediaType(TextFormat.CONTENT_TYPE_OPENMETRICS_100);
+		client.get().uri("/actuator/prometheus").accept(openMetrics).exchange().expectStatus().isOk().expectHeader()
+				.contentType(openMetrics).expectBody(String.class).value((body) -> assertThat(body)
+						.contains("counter1_total").contains("counter2_total").contains("counter3_total"));
+	}
+
 	@WebEndpointTest
 	void scrapeWithIncludedNames(WebTestClient client) {
 		client.get().uri("/actuator/prometheus?includedNames=counter1_total,counter2_total").exchange().expectStatus()