Await registration of http.server.requests meter

Previously, the test would make an HTTP request and, as soon as the
response was received, it would check the presence and value of the
http.server.requests meter. This create a race condition between the
meter being registered once the response had been flushed and the
meter's presence being checked. If the check won the race, the test
would fail.

This commit updates the test to wait for up to 5 seconds for the
meter to be present and have a count of 1, matching the single request
that has been made.

Fixes gh-23919

spring-boot-project/spring-boot-actuator-autoconfigure/pom.xml

@@ -546,6 +546,11 @@
 			<artifactId>aspectjrt</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.awaitility</groupId>
+			<artifactId>awaitility</artifactId>
+			<scope>test</scope>
+		</dependency>
 		<dependency>
 			<groupId>org.eclipse.jetty</groupId>
 			<artifactId>jetty-webapp</artifactId>

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/metrics/test/MetricsIntegrationTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.boot.actuate.autoconfigure.metrics.test;
 
+import java.time.Duration;
 import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
@@ -71,6 +72,7 @@ import org.springframework.web.bind.annotation.RestController;
 import org.springframework.web.client.RestTemplate;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.waitAtMost;
 import static org.springframework.test.web.client.ExpectedCount.once;
 import static org.springframework.test.web.client.match.MockRestRequestMatchers.method;
 import static org.springframework.test.web.client.match.MockRestRequestMatchers.requestTo;
@@ -110,7 +112,9 @@ class MetricsIntegrationTests {
 	@Test
 	void requestMappingIsInstrumented() {
 		this.loopback.getForObject("/api/people", Set.class);
-		assertThat(this.registry.get("http.server.requests").timer().count()).isEqualTo(1);
+		waitAtMost(Duration.ofSeconds(5)).untilAsserted(
+				() -> assertThat(this.registry.get("http.server.requests").timer().count()).isEqualTo(1));
+
 	}
 
 	@Test