sql: fix "rows written" metrics

[yuzefovich]

This commit fixes a bug that was introduced long time ago in 03c09a3. In particular, in that change we began tracking "rows written" for the purposes of the guardrails, and later this information was also added into statement statistics as well as extended to some metrics.

The bug was that we only check the first wrapped planNode for being a mutationPlanNode, but it's actually possible for a single planNodeToRowSource wrapper to wrap multiple planNodes at once. For example, if we have a mutation in the CTE, then we could have bufferNode -> insertNode -> valuesNode, all handled by a single wrapper. Previously, we would only check bufferNode, and since it's not a mutation planNode, we'd emit no "written" metrics.

This commit fixes the issue by traversing the planNode tree until we either reach the first not-wrapped planNode (i.e. it's handled by DistSQL separately) or reach the leaf nodes.

Note that this bug was further hidden by the fact that if we collect execution stats (which we do for stmt bundles and when the txn is sampled), then we wrap each planNode separately, so in those scenarios the behavior was correct.

Epic: None

Release note (bug fix): Previously, if a mutation was executed in a subquery (e.g. as a CTE), the "rows written" metrics like sql.statements.index_rows_written.count and
sql.statements.index_bytes_written.count might not be incremented correctly. This is now fixed.

[trunk-io]

😎 Merged successfully - details.

[cockroach-teamcity]

This change is 

[DrewKimball]

@DrewKimball reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on yuzefovich).

[yuzefovich]

TFTR!

/trunk merge

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merge conflict cherry-picking 684c04f to blathers/backport-release-25.4-167360

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