Add some logging to debug timestamp selection for REFRESH MVs

[ggevay]

Logging for https://github.com/MaterializeInc/database-issues/issues/8821
as discussed at
https://materializeinc.slack.com/archives/C0761MZ3QD9/p1733845868029119?thread_ts=1733597183.408179&cid=C0761MZ3QD9
so that we'll have more info the next time it happens in CI.

The placement of the log in appends.rs is @jkosh44's suggestion on Slack.

Motivation

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.