Align counter behaviour between metric exporters

The MetricCopyExporter has had the capability for a while to keep
track of counters internally. This change aligns that with the
PrefixMetricGroupExporter.

Fixes gh-5762

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

@@ -19,10 +19,13 @@ package org.springframework.boot.actuate.metrics.export;
 import java.util.Collection;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 
 import org.springframework.boot.actuate.metrics.Metric;
 import org.springframework.boot.actuate.metrics.reader.PrefixMetricReader;
 import org.springframework.boot.actuate.metrics.repository.MultiMetricRepository;
+import org.springframework.boot.actuate.metrics.writer.Delta;
 import org.springframework.boot.actuate.metrics.writer.PrefixMetricWriter;
 
 /**
@@ -38,6 +41,8 @@ public class PrefixMetricGroupExporter extends AbstractMetricExporter {
 
 	private final PrefixMetricWriter writer;
 
+	private ConcurrentMap<String, Long> counts = new ConcurrentHashMap<String, Long>();
+
 	private Set<String> groups = new HashSet<String>();
 
 	/**
@@ -88,7 +93,26 @@ public class PrefixMetricGroupExporter extends AbstractMetricExporter {
 
 	@Override
 	protected void write(String group, Collection<Metric<?>> values) {
-		this.writer.set(group, values);
+		if (group.contains("counter.")) {
+			for (Metric<?> value : values) {
+				this.writer.increment(group, calculateDelta(value));
+			}
+		}
+		else {
+			this.writer.set(group, values);
+		}
+	}
+
+	private Delta<?> calculateDelta(Metric<?> value) {
+		long delta = value.getValue().longValue();
+		Long old = this.counts.replace(value.getName(), delta);
+		if (old != null) {
+			delta = delta - old;
+		}
+		else {
+			this.counts.putIfAbsent(value.getName(), delta);
+		}
+		return new Delta<Long>(value.getName(), delta, value.getTimestamp());
 	}
 
 }

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

@@ -24,6 +24,7 @@ import org.junit.Test;
 import org.springframework.boot.actuate.metrics.Iterables;
 import org.springframework.boot.actuate.metrics.Metric;
 import org.springframework.boot.actuate.metrics.repository.InMemoryMetricRepository;
+import org.springframework.boot.actuate.metrics.writer.Delta;
 
 import static org.junit.Assert.assertEquals;
 
@@ -50,6 +51,15 @@ public class PrefixMetricGroupExporterTests {
 		assertEquals(1, Iterables.collection(this.writer.groups()).size());
 	}
 
+	@Test
+	public void countersIncremented() {
+		this.writer.increment("counter.foo", new Delta<Long>("bar", 1L));
+		this.reader.set(new Metric<Number>("counter.foo.bar", 1));
+		this.exporter.setGroups(Collections.singleton("counter.foo"));
+		this.exporter.export();
+		assertEquals(2L, this.writer.findAll("counter.foo").iterator().next().getValue());
+	}
+
 	@Test
 	public void unprefixedMetricsNotCopied() {
 		this.reader.set(new Metric<Number>("foo.bar", 2.3));