Track the number of buckets dropped.

Test: atest statsd_test
Change-Id: I56973d2c87aaed0f1c4a6908682cc54cda39eceb
diff --git a/cmds/statsd/src/metrics/DurationMetricProducer.cpp b/cmds/statsd/src/metrics/DurationMetricProducer.cpp
index 69bafc3..6c1c47b 100644
--- a/cmds/statsd/src/metrics/DurationMetricProducer.cpp
+++ b/cmds/statsd/src/metrics/DurationMetricProducer.cpp
@@ -444,6 +444,7 @@
 
 void DurationMetricProducer::dropDataLocked(const int64_t dropTimeNs) {
     flushIfNeededLocked(dropTimeNs);
+    StatsdStats::getInstance().noteBucketDropped(mMetricId);
     mPastBuckets.clear();
 }