Stop MetricsEndpoint from summing up same metrics

Update `MetricsEndpoint` so that only the first matching meter is used
when calculating the sum of of statistics.

Prior this this commit the endpoint would consider all Meters. This
caused incorrect statistics when multiple back-end systems were being
used since the registries contained in the `CompositeMeterRegistry`
would be  iterated, and the same effective metric would be counted more
than once.

Closes gh-14497

Author: pulkitmehra

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

@@ -16,7 +16,7 @@
 
 package org.springframework.boot.actuate.metrics;
 
-import java.util.ArrayList;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -81,15 +81,15 @@ private String getName(Meter meter) {
 	public MetricResponse metric(@Selector String requiredMetricName,
 			@Nullable List<String> tag) {
 		List<Tag> tags = parseTags(tag);
-		List<Meter> meters = new ArrayList<>();
-		collectMeters(meters, this.registry, requiredMetricName, tags);
+		Collection<Meter> meters = findFirstMatchingMeters(this.registry,
+				requiredMetricName, tags);
 		if (meters.isEmpty()) {
 			return null;
 		}
 		Map<Statistic, Double> samples = getSamples(meters);
 		Map<String, Set<String>> availableTags = getAvailableTags(meters);
 		tags.forEach((t) -> availableTags.remove(t.getKey()));
-		Meter.Id meterId = meters.get(0).getId();
+		Meter.Id meterId = meters.iterator().next().getId();
 		return new MetricResponse(requiredMetricName, meterId.getDescription(),
 				meterId.getBaseUnit(), asList(samples, Sample::new),
 				asList(availableTags, AvailableTag::new));
@@ -112,18 +112,25 @@ private Tag parseTag(String tag) {
 		return Tag.of(parts[0], parts[1]);
 	}
 
-	private void collectMeters(List<Meter> meters, MeterRegistry registry, String name,
+	private Collection<Meter> findFirstMatchingMeters(MeterRegistry registry, String name,
 			Iterable<Tag> tags) {
 		if (registry instanceof CompositeMeterRegistry) {
-			((CompositeMeterRegistry) registry).getRegistries()
-					.forEach((member) -> collectMeters(meters, member, name, tags));
+			return ((CompositeMeterRegistry) registry).getRegistries().stream()
+					.map((r) -> findFirstMatchingMeters(r, name, tags))
+					.filter((match) -> !match.isEmpty()).findFirst()
+					.orElse(Collections.emptyList());
+
 		}
 		else {
-			meters.addAll(registry.find(name).tags(tags).meters());
+			Collection<Meter> metersFound = registry.find(name).tags(tags).meters();
+			if (!metersFound.isEmpty()) {
+				return metersFound;
+			}
 		}
+		return Collections.emptyList();
 	}
 
-	private Map<Statistic, Double> getSamples(List<Meter> meters) {
+	private Map<Statistic, Double> getSamples(Collection<Meter> meters) {
 		Map<Statistic, Double> samples = new LinkedHashMap<>();
 		meters.forEach((meter) -> mergeMeasurements(samples, meter));
 		return samples;
@@ -138,7 +145,7 @@ private BiFunction<Double, Double, Double> mergeFunction(Statistic statistic) {
 		return Statistic.MAX.equals(statistic) ? Double::max : Double::sum;
 	}
 
-	private Map<String, Set<String>> getAvailableTags(List<Meter> meters) {
+	private Map<String, Set<String>> getAvailableTags(Collection<Meter> meters) {
 		Map<String, Set<String>> availableTags = new HashMap<>();
 		meters.forEach((meter) -> mergeAvailableTags(availableTags, meter));
 		return availableTags;

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

@@ -95,6 +95,55 @@ public void metricValuesAreTheSumOfAllTimeSeriesMatchingTags() {
 		assertThat(getCount(response)).hasValue(4.0);
 	}
 
+	@Test
+	public void findFirstMatchingMetersFromNestedRegistries() {
+		CompositeMeterRegistry composite = new CompositeMeterRegistry();
+		SimpleMeterRegistry reg1 = new SimpleMeterRegistry();
+		CompositeMeterRegistry reg2 = new CompositeMeterRegistry();
+		SimpleMeterRegistry reg3 = new SimpleMeterRegistry();
+
+		// 1st level nesting
+		composite.add(reg1);
+
+		// 2st level nesting
+		reg2.add(reg3);
+		composite.add(reg2);
+
+		// 2nd level registry has metrics
+		reg3.counter("cache", "result", "hit", "host", "1").increment(2);
+		reg3.counter("cache", "result", "miss", "host", "1").increment(2);
+		reg3.counter("cache", "result", "hit", "host", "2").increment(2);
+
+		MetricsEndpoint endpoint = new MetricsEndpoint(composite);
+
+		MetricsEndpoint.MetricResponse response = endpoint.metric("cache",
+				Collections.emptyList());
+		assertThat(response.getName()).isEqualTo("cache");
+		assertThat(availableTagKeys(response)).containsExactly("result", "host");
+		assertThat(getCount(response)).hasValue(6.0);
+
+		response = endpoint.metric("cache", Collections.singletonList("result:hit"));
+		assertThat(availableTagKeys(response)).containsExactly("host");
+		assertThat(getCount(response)).hasValue(4.0);
+	}
+
+	@Test
+	public void matchingMeterNotFoundInNestedRegistries() {
+		CompositeMeterRegistry composite = new CompositeMeterRegistry();
+		CompositeMeterRegistry reg2 = new CompositeMeterRegistry();
+		SimpleMeterRegistry reg3 = new SimpleMeterRegistry();
+
+		// nested registries
+		reg2.add(reg3);
+		composite.add(reg2);
+
+		MetricsEndpoint endpoint = new MetricsEndpoint(composite);
+
+		MetricsEndpoint.MetricResponse response = endpoint.metric("invalid.metric.name",
+				Collections.emptyList());
+		assertThat(response).isNull();
+	}
+
 	@Test
 	public void metricTagValuesAreDeduplicated() {
 		this.registry.counter("cache", "host", "1", "region", "east", "result", "hit");