Track the number of buckets dropped.

Test: atest statsd_test
Change-Id: I56973d2c87aaed0f1c4a6908682cc54cda39eceb
diff --git a/cmds/statsd/src/stats_log.proto b/cmds/statsd/src/stats_log.proto
index 5b91482..73aab48 100644
--- a/cmds/statsd/src/stats_log.proto
+++ b/cmds/statsd/src/stats_log.proto
@@ -418,6 +418,7 @@
       optional int64 bad_value_type = 5;
       optional int64 condition_change_in_next_bucket = 6;
       optional int64 invalidated_bucket = 7;
+      optional int64 bucket_dropped = 8;
     }
     repeated AtomMetricStats atom_metric_stats = 17;