adapter: add `mz_statement_logging_record_count` metric

[ParkMyCar]

This PR adds the mz_statement_logging_record_count prometheus metric to track the total number of statements we record in the recent activity log, and whether or not that statement was actually sampled.

Motivation

Chatted about this in the incident response working group, if we change the sampling rate for a user we want a more concrete way to see that the change took effect.

Checklist

• [x] This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• [x] This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• [x] If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• [x] If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• [x] If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.