[logging] Don't render large VALUES when redacting

[mgree]

We attach SQL statements to spans in coord::handle_execute_inner, using stmt.to_ast_string_redacted(). In the case of a query with 100000 numbers in a VALUES clause, we'll log 100000 redacted numbers.

There's no point in recording that many redacted values. When we're doing redaction, we'll only log the first 20 values, leaving a comment indicating how many more there ought to be.

(This could be refined: if there are 21 VALUES that are meaningful, non-redacted expressions, we'll drop the last one. We could instead render the first 20 redacted values, and drop them after that. Such a policy would require a bit more nuance than this PR.)

Motivation

• This PR fixes a recognized bug.
  https://github.com/MaterializeInc/database-issues/issues/8832#event-15647939462

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.

[ParkMyCar]

LGTM