adapter: Add result_rows_first_to_last_byte_seconds metric

[ggevay]

Add histogram metric for time between first result row returned and last result row returned. (This is part of #32504, although it's not yet mentioned in the design doc.)

This is more involved than my other similar PRs from yesterday, because the control flow for returning results is a bit complex, with several variations in various situations, see tests. (I wanted to also add a test for a FETCH that has a timeout, but it seems FETCH's timeout is currently not working: https://github.com/MaterializeInc/database-issues/issues/9354 )

Nightly: https://buildkite.com/materialize/nightly/builds/12231

Motivation

• This PR adds a feature that has not yet been specified.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• 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.
• 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).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[ggevay]

(I've checked that the only existing metric that this change affected is one of the metrics that I added yesterday: mz_pgwire_message_processing_seconds. I didn't know that there is this limitation when I added that metric, so what I put into the histogram_seconds_buckets was not taking effect. Now this is fixed.)