feat(vdiff): add summary_only option to vdiff show

[pedroalb]

Description

Adds a summary_only option to vdiff show that omits the large per-table row-sample arrays from each target's response, returning the vdiff and per-table state, has_mismatch, rows_compared, and the report's scalar counters.

The per-table report column holds a JSON DiffReport containing both scalar counters (ProcessedRows, MatchingRows, MismatchedRows, ExtraRowsSource, ExtraRowsTarget) and sampled row data in the MismatchedRowsSample, ExtraRowsSourceSample, and ExtraRowsTargetSample arrays. The sample arrays carry actual row data, including large blob/JSON columns, and vdiff show aggregates them across every target shard, so for blob-heavy tables the response can exceed gRPC message limits.

When summary_only is set, each target primary strips just those sample arrays via JSON_REMOVE(vdt.report, '$.MismatchedRowsSample', '$.ExtraRowsSourceSample', '$.ExtraRowsTargetSample') while preserving the scalar counters. This keeps the reported counts accurate (so has_mismatch can never disagree with MismatchedRows) and drops only the unbounded, blob-carrying part of the report, letting callers that only need a continue/cancel decision (e.g. the resharder) avoid transferring the samples. JSON_REMOVE returns NULL when the report is NULL (a LEFT JOIN with no matching vdiff_table row), matching the plain-column behavior.

The option is threaded end to end:

• VDiffShowRequest.summary_only (vtctld API)
• forwarded onto each tablet's VDiffRequest.Options.ReportOptions.summary_only before the fan-out, so the target primaries strip the samples at the source
• getVDiffSummary selects the sample-stripping report expression via a single DRY summary query (only the report select-expression varies)
• a --summary-only flag on the vtctldclient vdiff show command

The new proto fields are additive (new field numbers, optional), so mixed-version vtctld/vttablet deployments remain safe and default to existing behavior. Note the size reduction only takes effect once the VDiff target primaries run this build, since the sample stripping happens on the tablet.

Note on the design

An earlier revision blanked the whole report to a literal '{}'. That was changed because it (a) made BuildSummary parse zero for every counter, so the summary could show HasMismatch: true alongside MismatchedRows: 0, and (b) turned the LEFT-JOIN NULL report into a non-NULL '{}'. Stripping only the sample arrays with JSON_REMOVE avoids both while still removing the large payload. This matches the approach taken upstream in vitessio#20870.

Related Issue(s)

Internal: reduces VDiffShow response size for blob-heavy keyspaces that were exceeding gRPC message limits during resharding. Upstreamed as vitessio#20870 (issue vitessio#20869).

Testing

Unit test. TestVDiffSummaryQuery (go/vt/vttablet/tabletmanager/vdiff/action_test.go) asserts structurally (not by brittle full-string match):

• both variants preserve the %a bind placeholders so the query stays parameterized;
• the non-summary query returns the stored report as-is and does not use JSON_REMOVE;
• the summary-only query strips exactly the three sample arrays;
• the two variants are identical after swapping the report select-expression, so no other summary column or clause can silently differ.

MySQL semantics verified on 8.0.46 and 5.7.44 (Docker), including the full summary query against a populated _vt.vdiff/vdiff_table schema:

• sample arrays removed, all scalar counters preserved;
• a report with no sample arrays is returned unchanged (JSON_REMOVE of absent paths is a no-op);
• the LEFT JOIN-with-no-row case returns a NULL report on both versions.

Manual verification on a dev/staging cluster via vtctl-slack VDiff ... show <uuid>:

• default (text) output: unchanged (state + per-table counts).
• --format=json: full per-table Reports present (existing behavior).
• --format=json --summary-only: the sampled-row arrays are omitted while State, HasMismatch, RowsCompared, and the per-table counters remain populated.

Confirmed the --summary-only flag is present and wired through vtctl-slack VDiff show --help.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

No DB migrations. Roll out to VDiff target primaries (vttablet) to realize the response-size reduction; vtctld/CLI changes are safe in any version-skew order and default to current behavior. The JSON_REMOVE JSON-path functions are available on MySQL 5.7 and 8.0 (verified on both).