Adds option to drop small buckets for statsd.

We notice that some of the pulled metrics have a ton of data, and
during app upgrades, we're forming partial buckets that represent
small periods of time but require many bytes of data. We now have an
option to drop these buckets that are too short. Note that we still
have to pull the data to keep the metrics for the next bucket
correct. We include a new field in the value and gauge metric outputs
so that it's easy to tell when a bucket was dropped.

We drop the partial buckets also from anomaly detection since we
should be computing anomalies from the same data that is reported.

Test: Added unit-tests for value and gauge metrics.
Bug: 77925710
Change-Id: Ic370496377c6afd380e02278a6c1ed8b521a2731