Add AggregateMetricReader able to aggregate counters and gauges

Different physical sources for the same logical metric just need to
publish them with a period-separated prefix, and this reader will
aggregate (by truncating the metric names, dropping the prefix).

Very useful (for instance) if multiple application instances are
feeding to a central (e.g. redis) repository and you want to
display the results. Useful in conjunction with a
MetricReaderPublicMetrics for hooking up to the /metrics endpoint.

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

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2012-2015 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.aggregate;
+
+import java.util.Date;
+
+import org.junit.Test;
+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;
+import static org.junit.Assert.assertNull;
+
+/**
+ * @author Dave Syer
+ */
+public class AggregateMetricReaderTests {
+
+	private InMemoryMetricRepository source = new InMemoryMetricRepository();
+
+	private AggregateMetricReader reader = new AggregateMetricReader(this.source);
+
+	@Test
+	public void writeAndReadDefaults() {
+		this.source.set(new Metric<Double>("foo.bar.spam", 2.3));
+		assertEquals(2.3, this.reader.findOne("aggregate.spam").getValue());
+	}
+
+	@Test
+	public void writeAndReadLatestValue() {
+		this.source.set(new Metric<Double>("foo.bar.spam", 2.3, new Date(100L)));
+		this.source.set(new Metric<Double>("oof.rab.spam", 2.4, new Date(0L)));
+		assertEquals(2.3, this.reader.findOne("aggregate.spam").getValue());
+	}
+
+	@Test
+	public void writeAndReadExtraLong() {
+		this.source.set(new Metric<Double>("blee.foo.bar.spam", 2.3));
+		this.reader.setTruncateKeyLength(3);
+		assertEquals(2.3, this.reader.findOne("aggregate.spam").getValue());
+	}
+
+	@Test
+	public void onlyPrefixed() {
+		this.source.set(new Metric<Double>("foo.bar.spam", 2.3));
+		assertNull(this.reader.findOne("spam"));
+	}
+
+	@Test
+	public void incrementCounter() {
+		this.source.increment(new Delta<Long>("foo.bar.counter.spam", 2L));
+		this.source.increment(new Delta<Long>("oof.rab.counter.spam", 3L));
+		assertEquals(5L, this.reader.findOne("aggregate.counter.spam").getValue());
+	}
+
+	@Test
+	public void countGauges() {
+		this.source.set(new Metric<Double>("foo.bar.spam", 2.3));
+		this.source.set(new Metric<Double>("oof.rab.spam", 2.4));
+		assertEquals(1, this.reader.count());
+	}
+
+	@Test
+	public void countGaugesAndCounters() {
+		this.source.set(new Metric<Double>("foo.bar.spam", 2.3));
+		this.source.set(new Metric<Double>("oof.rab.spam", 2.4));
+		this.source.increment(new Delta<Long>("foo.bar.counter.spam", 2L));
+		this.source.increment(new Delta<Long>("oof.rab.counter.spam", 3L));
+		assertEquals(2, this.reader.count());
+	}
+
+}