Aggregate max statistics in metrics endpoint with Double#max

See gh-11852

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

@@ -121,7 +121,11 @@ public class MetricsEndpoint {
 
 	private void mergeMeasurements(Map<Statistic, Double> samples, Meter meter) {
 		meter.measure().forEach((measurement) -> samples.merge(measurement.getStatistic(),
-				measurement.getValue(), Double::sum));
+				measurement.getValue(), mergeFunction(measurement.getStatistic())));
+	}
+
+	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) {

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

@@ -18,6 +18,7 @@ package org.springframework.boot.actuate.metrics;
 
 import java.util.Collections;
 import java.util.Optional;
+import java.util.concurrent.TimeUnit;
 import java.util.stream.Stream;
 
 import io.micrometer.core.instrument.MeterRegistry;
@@ -129,6 +130,31 @@ public class MetricsEndpointTests {
 		assertThat(response).isNull();
 	}
 
+	@Test
+	public void maxAggregation() {
+		SimpleMeterRegistry reg = new SimpleMeterRegistry();
+		reg.timer("timer", "k", "v1").record(1, TimeUnit.SECONDS);
+		reg.timer("timer", "k", "v2").record(2, TimeUnit.SECONDS);
+
+		assertMetricHasStatisticEqualTo(reg, "timer", Statistic.MAX, 2.0);
+	}
+
+	@Test
+	public void countAggregation() {
+		SimpleMeterRegistry reg = new SimpleMeterRegistry();
+		reg.counter("counter", "k", "v1").increment();
+		reg.counter("counter", "k", "v2").increment();
+
+		assertMetricHasStatisticEqualTo(reg, "counter", Statistic.COUNT, 2.0);
+	}
+
+	private void assertMetricHasStatisticEqualTo(MeterRegistry registry, String metricName, Statistic stat, Double value) {
+		MetricsEndpoint endpoint = new MetricsEndpoint(registry);
+		assertThat(endpoint.metric(metricName, Collections.emptyList()).getMeasurements()
+				.stream().filter(s -> s.getStatistic().equals(stat)).findAny())
+				.hasValueSatisfying(s -> assertThat(s.getValue()).isEqualTo(value));
+	}
+
 	private Optional<Double> getCount(MetricsEndpoint.MetricResponse response) {
 		return response.getMeasurements().stream()
 				.filter((ms) -> ms.getStatistic().equals(Statistic.COUNT)).findAny()