Improve diagnostics in PublicMetricsAutoConfigurationTests

multipleCacheManagers fails on the CI server but does not fail
locally. This commit updates assertMetrics to include information
about the actual metrics so it's clear why the expectation was not
met.

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/PublicMetricsAutoConfigurationTests.java

@@ -56,9 +56,10 @@
 
 import com.zaxxer.hikari.HikariDataSource;
 
+import static org.hamcrest.Matchers.hasKey;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.assertThat;
 import static org.junit.Assert.fail;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.mock;
@@ -236,7 +237,7 @@ private void assertMetrics(Collection<Metric<?>> metrics, String... keys) {
 			content.put(metric.getName(), metric.getValue());
 		}
 		for (String key : keys) {
-			assertTrue("Key '" + key + "' was not found", content.containsKey(key));
+			assertThat(content, hasKey(key));
 		}
 	}