Implement a logic to rate limitted flush statsd with by checking the
size of each MetricsProducer's. The implementation of byteSize() method
is still TBD as it depends on migration to ProtoOutputStream.

Test: statsd, statsd_test
Change-Id: I966606044d7cb814dabe94192bacecad91f28177
diff --git a/cmds/statsd/src/metrics/MetricProducer.h b/cmds/statsd/src/metrics/MetricProducer.h
index 496b145..80eb527 100644
--- a/cmds/statsd/src/metrics/MetricProducer.h
+++ b/cmds/statsd/src/metrics/MetricProducer.h
@@ -64,6 +64,8 @@
         return mConditionSliced;
     };
 
+    virtual size_t byteSize() = 0;
+
 protected:
     const uint64_t mStartTimeNs;