kafka: fix metrics

[3AceShowHand]

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Issue Number: close #xxx

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Check List

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Summary by CodeRabbit

• Refactor
  • Simplified checkpoint delivery processing across cloud storage, Kafka, and Pulsar sinks.
  • Removed checkpoint message count and duration metrics.
  • Streamlined message queue and sink monitoring while preserving checkpoint encoding, publishing, and delivery behavior.
  • Internal Kafka metric visibility was reduced without changing broker metric values, labels, collection, or cleanup behavior.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: bfb2b90a-826f-4b9a-9ab2-cdb0d16085df

📥 Commits

Reviewing files that changed from the base of the PR and between 918306c and 617db02.

📒 Files selected for processing (2)

• pkg/sink/kafka/metrics.go
• pkg/sink/kafka/metrics_collector.go

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📝 Walkthrough

Walkthrough

The change removes checkpoint message metrics from cloud storage, Kafka, and Pulsar sinks. It removes unused MQ metric definitions and registrations. It also makes Kafka broker metric gauges package-private while preserving their collection and cleanup behavior.

Changes

Sink metric cleanup

Layer / File(s)	Summary
Remove metric definitions and registry wiring pkg/metrics/sink.go, downstreamadapter/sink/metrics/mq.go	Removed checkpoint metric declarations, registrations, and unused MQ metric definitions. InitMQMetrics retains synchronized registry storage.
Remove checkpoint instrumentation downstreamadapter/sink/cloudstorage/sink.go, downstreamadapter/sink/kafka/sink.go, downstreamadapter/sink/pulsar/sink.go	Removed checkpoint metric initialization, timing, label cleanup, count updates, and duration observations. Checkpoint encoding and delivery remain unchanged.
Restrict Kafka metric visibility pkg/sink/kafka/metrics.go, pkg/sink/kafka/metrics_collector.go	Renamed Kafka broker metric gauges to package-private identifiers. Registration, collection, labels, and cleanup remain unchanged.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• pingcap/ticdc#5801: Continues the sink statistics refactor by removing obsolete metric definitions and usages.
• pingcap/ticdc#5866: Refactors sink-specific Prometheus metrics in related code paths.

Suggested reviewers: hongyunyan, wk989898

Poem

A rabbit trimmed metrics from the trail,
Removed old counters from each sail.
Kafka gauges now hide from view,
Checkpoints still reach their destination true.
The registry stays locked; hops continue.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description retains the unfilled template and provides no problem statement, implementation details, testing information, impact assessment, or release note.	Complete each required section, replace the placeholder issue reference, document testing and impact, and add a release note or None.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the Kafka metrics change addressed by the pull request.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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• Create PR with unit tests

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