Duration tracker optimization.

* Avoid querying sliced condition for stop/stopAll events for duration metric.
* Avoid extracting the internal dimension key when it is identical to the what dimension.

Test: statsd test
Change-Id: I664e8d3b1a68960d05c9ce4789caefb60b1ab502
diff --git a/cmds/statsd/src/metrics/MetricProducer.h b/cmds/statsd/src/metrics/MetricProducer.h
index 83e1740..2bf6241 100644
--- a/cmds/statsd/src/metrics/MetricProducer.h
+++ b/cmds/statsd/src/metrics/MetricProducer.h
@@ -233,7 +233,7 @@
             const LogEvent& event) = 0;
 
     // Consume the parsed stats log entry that already matched the "what" of the metric.
-    void onMatchedLogEventLocked(const size_t matcherIndex, const LogEvent& event);
+    virtual void onMatchedLogEventLocked(const size_t matcherIndex, const LogEvent& event);
 
     mutable std::mutex mMutex;
 };