Adding the APITimer

The Timer object in the yammer library is used to regularly check a
histogram.

The APITimer is a Timer that uses the APIHistogram instead that in it
self calls the API to get its values.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>

src/main/java/com/yammer/metrics/core/APITimer.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2015 Cloudius Systems
+ *
+ */
+package com.yammer.metrics.core;
+
+import java.lang.reflect.Field;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+
+import com.yammer.metrics.core.Histogram.SampleType;
+
+/**
+ * A timer metric which aggregates timing durations and provides duration
+ * statistics, plus throughput statistics via {@link Meter}.
+ */
+public class APITimer extends Timer {
+
+    public APITimer(String url, ScheduledExecutorService tickThread,
+            TimeUnit durationUnit, TimeUnit rateUnit) {
+        super(tickThread, durationUnit, rateUnit);
+        setHistogram(url);
+    }
+
+    public APITimer(String url, ScheduledExecutorService tickThread,
+            TimeUnit durationUnit, TimeUnit rateUnit, Clock clock) {
+        super(tickThread, durationUnit, rateUnit, clock);
+        setHistogram(url);
+    }
+
+    private void setHistogram(String url) {
+        Field histogram;
+        try {
+            histogram = Timer.class.getDeclaredField("histogram");
+            histogram.setAccessible(true);
+            histogram.set(this, new APIHistogram(url, SampleType.BIASED));
+        } catch (NoSuchFieldException | SecurityException
+                | IllegalArgumentException | IllegalAccessException e) {
+            // TODO Auto-generated catch block
+            e.printStackTrace();
+        }
+    }
+
+}