This CL adds Truth framework subjects to some metrics in the Stackdriver metrics library, in a contrib subpackage. It doesn't deal with gauge metrics, and for event metrics, the assertions can only be that a metric has or does not have a distribution for a particular set of label values. Asserting more fine-grained propositions regarding the distribution will require a distribution subject.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=149112692