This change enforces that IncrementableMetrics should only monotonically
increase in value, and adds a new increment() method to increment by one, which is slightly faster than incrementBy (due to a lack of non-negative checking) in the common case that the counter should only be incremented by one.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130578421
f10a7d8fb0