Update metrics endpoint to return list of distinct names

Closes gh-10336

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/MetricsEndpoint.java

@@ -53,7 +53,7 @@ public class MetricsEndpoint {
 	@ReadOperation
 	public Map<String, List<String>> listNames() {
 		return Collections.singletonMap("names", this.registry.getMeters().stream()
-				.map(this::getMeterIdName).collect(Collectors.toList()));
+				.map(this::getMeterIdName).distinct().collect(Collectors.toList()));
 	}
 
 	private String getMeterIdName(Meter meter) {

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/MetricsEndpointTests.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2012-2017 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;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+
+import io.micrometer.core.instrument.Meter;
+import io.micrometer.core.instrument.Meter.Id;
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.simple.SimpleCounter;
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link MetricsEndpoint}.
+ *
+ * @author Andy Wilkinson
+ */
+public class MetricsEndpointTests {
+
+	private final MeterRegistry registry = mock(MeterRegistry.class);
+
+	private final MetricsEndpoint endpoint = new MetricsEndpoint(this.registry);
+
+	@Test
+	public void listNamesHandlesEmptyListOfMeters() {
+		given(this.registry.getMeters()).willReturn(Arrays.asList());
+		Map<String, List<String>> result = this.endpoint.listNames();
+		assertThat(result).containsOnlyKeys("names");
+		assertThat(result.get("names")).isEmpty();
+	}
+
+	@Test
+	public void listNamesProducesListOfUniqueMeterNames() {
+		List<Meter> meters = Arrays.asList(createCounter("com.example.foo"),
+				createCounter("com.example.bar"), createCounter("com.example.foo"));
+		given(this.registry.getMeters()).willReturn(meters);
+		Map<String, List<String>> result = this.endpoint.listNames();
+		assertThat(result).containsOnlyKeys("names");
+		assertThat(result.get("names")).containsOnlyOnce("com.example.foo",
+				"com.example.bar");
+	}
+
+	private Meter createCounter(String name) {
+		return new SimpleCounter(createMeterId(name));
+	}
+
+	private Id createMeterId(String name) {
+		Id id = mock(Id.class);
+		given(id.getName()).willReturn(name);
+		return id;
+	}
+
+}

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/MetricsEndpointWebIntegrationTests.java

@@ -54,7 +54,7 @@ public class MetricsEndpointWebIntegrationTests {
 				.uri("/application/metrics").exchange().expectStatus().isOk()
 				.expectBody(String.class).returnResult().getResponseBody();
 		Map<String, List<String>> names = this.mapper.readValue(responseBody, Map.class);
-		assertThat(names.get("names")).contains("jvm.memory.used");
+		assertThat(names.get("names")).containsOnlyOnce("jvm.memory.used");
 	}
 
 	@Test