Auto-configure observations for RestClients

Closes gh-38500

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

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2012-2023 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.web.client;
+
+import io.micrometer.observation.ObservationRegistry;
+import io.micrometer.observation.tck.TestObservationRegistry;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.client.observation.DefaultClientRequestObservationConvention;
+import org.springframework.web.client.RestClient;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ObservationRestClientCustomizer}.
+ *
+ * @author Brian Clozel
+ * @author Moritz Halbritter
+ */
+class ObservationRestClientCustomizerTests {
+
+	private static final String TEST_METRIC_NAME = "http.test.metric.name";
+
+	private final ObservationRegistry observationRegistry = TestObservationRegistry.create();
+
+	private final RestClient.Builder restClientBuilder = RestClient.builder();
+
+	private final ObservationRestClientCustomizer customizer = new ObservationRestClientCustomizer(
+			this.observationRegistry, new DefaultClientRequestObservationConvention(TEST_METRIC_NAME));
+
+	@Test
+	void shouldCustomizeObservationConfiguration() {
+		this.customizer.customize(this.restClientBuilder);
+		assertThat(this.restClientBuilder).hasFieldOrPropertyWithValue("observationRegistry", this.observationRegistry);
+		assertThat(this.restClientBuilder).extracting("observationConvention")
+			.isInstanceOf(DefaultClientRequestObservationConvention.class)
+			.hasFieldOrPropertyWithValue("name", TEST_METRIC_NAME);
+	}
+
+}