Sort names alphabetically in metrics list response

Closes gh-19934

Author: wilkinsona

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

@@ -1,5 +1,5 @@
 /*
- * 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.
@@ -21,10 +21,10 @@
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedHashMap;
-import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.TreeSet;
 import java.util.function.BiFunction;
 import java.util.stream.Collectors;
 
@@ -58,7 +58,7 @@ public MetricsEndpoint(MeterRegistry registry) {
 
 	@ReadOperation
 	public ListNamesResponse listNames() {
-		Set<String> names = new LinkedHashSet<>();
+		Set<String> names = new TreeSet<>();
 		collectNames(names, this.registry);
 		return new ListNamesResponse(names);
 	}

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

@@ -1,5 +1,5 @@
 /*
- * 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.
@@ -54,11 +54,16 @@ void listNamesHandlesEmptyListOfMeters() {
 
 	@Test
 	void listNamesProducesListOfUniqueMeterNames() {
-		this.registry.counter("com.example.foo");
-		this.registry.counter("com.example.bar");
-		this.registry.counter("com.example.foo");
+		this.registry.counter("com.example.alpha");
+		this.registry.counter("com.example.charlie");
+		this.registry.counter("com.example.bravo");
+		this.registry.counter("com.example.delta");
+		this.registry.counter("com.example.delta");
+		this.registry.counter("com.example.echo");
+		this.registry.counter("com.example.bravo");
 		MetricsEndpoint.ListNamesResponse result = this.endpoint.listNames();
-		assertThat(result.getNames()).containsOnlyOnce("com.example.foo", "com.example.bar");
+		assertThat(result.getNames()).containsExactly("com.example.alpha", "com.example.bravo", "com.example.charlie",
+				"com.example.delta", "com.example.echo");
 	}
 
 	@Test
@@ -71,7 +76,7 @@ void listNamesRecursesOverCompositeRegistries() {
 		reg1.counter("counter1").increment();
 		reg2.counter("counter2").increment();
 		MetricsEndpoint endpoint = new MetricsEndpoint(composite);
-		assertThat(endpoint.listNames().getNames()).containsOnly("counter1", "counter2");
+		assertThat(endpoint.listNames().getNames()).containsExactly("counter1", "counter2");
 	}
 
 	@Test