stats: add support for cardinality

We are evaluating the cost to add cardinality for stats, the step after evaluation is to design ease to use apis, and then getting it implemented.

The issue names no files, tests, or entry points. Begin by evaluating the cost of adding cardinality to stats, then define ease-of-use APIs before implementation. Done means the evaluation, API design, and implementation work described in the issue are complete.

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