APITimer: sum should return a value and values are in ns

When removing the pull based timers in the API the sum method in the
APITimer was left stubed by mistake.

This patch take the sum from the histogram as it should be.

Another missed changes are the units, in the yammer library the Timer
does unit conversion before returning the values.

This patch takes the unit conversion from the yammer library to be
compatible.

Signed-off-by: Amnon Heiman <amnon@scylladb.com>
Message-Id: <1464173726-7482-1-git-send-email-amnon@scylladb.com>

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

@@ -25,6 +25,10 @@ public class APITimer extends Timer {
     final APIHistogram histogram;
     APIClient c = new APIClient();
 
+    private double convertFromNS(double ns) {
+        return ns / TimeUnit.NANOSECONDS.convert(1, durationUnit);
+    }
+
     String url;
 
     public APITimer(String url, ScheduledExecutorService tickThread,
@@ -52,31 +56,31 @@ public class APITimer extends Timer {
     @Override
     public double max() {
         update_fields();
-        return histogram.max();
+        return convertFromNS(histogram.max());
     }
 
     @Override
     public double min() {
         update_fields();
-        return histogram.min();
+        return convertFromNS(histogram.min());
     }
 
     @Override
     public double mean() {
         update_fields();
-        return histogram.mean();
+        return convertFromNS(histogram.mean());
     }
 
     @Override
     public double stdDev() {
         update_fields();
-        return histogram.stdDev();
+        return convertFromNS(histogram.stdDev());
     }
 
     @Override
     public double sum() {
         update_fields();
-        return 0;
+        return convertFromNS(histogram.sum());
     }
 
     @Override