pkg/plugin: pad missing _time rows in instant log frames

[immanuwell]

Related issue: n/a

Describe Your Changes

Fix an edge case in instant log parsing.

If VictoriaLogs returns mixed rows where one row has _time and another one does not, the backend built a malformed frame with uneven field lengths, so Grafana just bails out.

This patch pads missing _time per row with the same fallback time already used when _time is missing for the whole response. Small fix, no extra behavior changes.

Repro:

1. Add this fixture:
   {"_time":"2024-02-20T14:04:27Z","_msg":"123","_stream":"{app=\"test\"}"}
{"_msg":"456","_stream":"{app=\"test\"}"}
2. Run go test ./pkg/plugin -run Test_parseInstantResponse -count=1
3. Before this patch it fails with frame has different field lengths
4. With this patch it passes

Why this is real:
VictoriaLogs docs say logs without _time are ingested with the current timestamp, so mixed rows are totally possible in practice, not some weird unreachable corner.

Checklist

• My change adheres to VictoriaMetrics contributing guidelines.
• My change adheres to VictoriaMetrics development goals.

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Summary by cubic

Pads missing _time values per row in instant log responses to prevent malformed frames and Grafana errors when rows mix with and without _time.

• Bug Fixes
  • In parseInstantResponse, use a single fallbackTime from nowFunc() to fill missing per-row _time, and reuse it when the whole response lacks _time.
  • Added a test and fixture for mixed rows to ensure equal field lengths and valid frames.
  • Updated changelog entry to document the fix.

^{Written for commit c634834. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}