Add API to let metrics directly drop data without writing to an output.

+ Metrics will do flushIfNeeded() to correctly move the clock and informing
  AnomalyTracker the past bucket info, and then clear past buckets.

+ We will still keep the current bucket data for the validity of the future metrics.

Bug: 70571383
Test: statsd_test
Change-Id: Ib13c45574974e7b4e82bd8f305091dc93bda76f5