Allow override of scheduler in ScheduledReporter (#2931)

#1524 introduced a fix to queuing issues and ends up doing atFixedDelay rather than atFixedRate.

Issues with binning exist in Graphite (and probably others).

See #2664

metrics-core/src/main/java/com/codahale/metrics/ScheduledReporter.java

@@ -166,7 +166,19 @@ synchronized void start(long initialDelay, long period, TimeUnit unit, Runnable
             throw new IllegalArgumentException("Reporter already started");
         }
 
-        this.scheduledFuture = executor.scheduleWithFixedDelay(runnable, initialDelay, period, unit);
+        this.scheduledFuture = getScheduledFuture(initialDelay, period, unit, runnable);
+    }
+
+
+    /**
+     * Schedule the task, and return a future.
+     * The current implementation uses scheduleWithFixedDelay, replacing scheduleWithFixedRate. This avoids queueing issues, but may
+     * cause some reporters to skip metrics, as scheduleWithFixedDelay introduces a growing delta from the original start point.
+     *
+     * Overriding this in a subclass to revert to the old behavior is permitted.
+     */
+    protected ScheduledFuture<?> getScheduledFuture(long initialDelay, long period, TimeUnit unit, Runnable runnable) {
+        return executor.scheduleWithFixedDelay(runnable, initialDelay, period, unit);
     }
 
     /**