The concrete implementation of a Metric is not of importance when asserting on the values it contains. Therefore this CL removes Metric<T> as a type parameter of AbstractMetricSubject. As a result the two implementations of the abstract subject can be used on any Metric<Long> and Metric<Distribution>, respectively.
Also migrate to Subject.Factory from deprecated SubjectFactory.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171012012