persist: remove CmdMetrics::run_cmd

[pH14]

Clarifies our metrics usage that cas_mismatch isn't incremented with run_cmd usage. Alternatively, we could just inline these counters to the one use of run_cmd, being init_state

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[danhhz]

given that init_state's treatment of mismatches is unique, inlining it is appealing, but it would certainly be a lot noisier inline. on the balance, I'm definitely okay either way