This CL also adds IncrementableMetric#reset() methods to allow resetting the
value and start timestamp of IncrementableMetrics.
This is necessary because some backends, like Stackdriver, use non-monotonic
changes in cumulative metric values to detect timeseries restarts. Tracking and
re-setting the start timestamp allows users to track mostly monotonic metrics
which may have non-monotonic discontinuities.
See https://cloud.google.com/monitoring/api/ref_v3/rest/v3/TimeSeries#Point for
more details.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130795229