switch process_metrics to take a callback function

lib/process_metrics.js

@@ -1,4 +1,4 @@
-var process_metrics = function (metrics, flushInterval) {
+var process_metrics = function (metrics, flushInterval, ts, flushCallback) {
     var starttime = Date.now();
     var key;
     var counter_rates = {};
@@ -81,8 +81,7 @@ var process_metrics = function (metrics, flushInterval) {
     //add processed metrics to the metrics_hash
     metrics.counter_rates = counter_rates;
     metrics.timer_data = timer_data;
-
-    return metrics;
+    flushCallback(ts, metrics);
   }
 
 exports.process_metrics = process_metrics

stats.js

@@ -71,10 +71,11 @@ function flushMetrics() {
     }
   });
 
-  metrics_hash = pm.process_metrics(metrics_hash, flushInterval)
+  pm.process_metrics(metrics_hash, flushInterval, time_stamp, function emitFlush() {
+    // Flush metrics to each backend.
+    backendEvents.emit('flush', time_stamp, metrics_hash);
+  });
 
-  // Flush metrics to each backend.
-  backendEvents.emit('flush', time_stamp, metrics_hash);
 };
 
 var stats = {