Log bucket drop reasons for ValueMetric and GaugeMetric

- For all cases where ValueMetric and GaugeMetric drop buckets, we will
log the reason for the drop and the timestamp of the drop event. All the
cases where ValueMetric drops buckets is documented in
go/valuemetric-bucketdropping. GaugeMetric only drops buckets when the
size of the bucket is less than the specified minimum bucket size.
- Dropped/skipped bucket data structures have been moved to
MetricProducer so all metrics can potentially log bucket drop cases.
- Added unit tests for ValueMetricProducer and GaugeMetricProducer to
check that all bucket drop cases can be logged and the number of drop
events is capped at 10 for one skipped bucket.
- ***Could not test drops due to a pull delay because pull delays are
measured by real elapsed time in #accumulateEvents.

Test: bit statsd_test:*
Bug: 145836670
Bug: 140421050
Bug: 146082150
Bug: 146082276
Change-Id: I0197660bf89837162330aeaafb5397c7328e3993