add an option to manually set value sent to StatsD instead of using the difference by default

Author: Chen Dong

src/diamond/handler/stats_d.py

@@ -132,6 +132,9 @@ def _send(self):
                 value = metric.raw_value
                 if metric.path in self.old_values:
                     value = value - self.old_values[metric.path]
+                # provide an option for caller to manually set the value
+                elif metric.value:
+                    value = metric.value
                 self.old_values[metric.path] = metric.raw_value
 
                 if hasattr(statsd, 'StatsClient'):