kafka: fix metrics

[3AceShowHand]

What problem does this PR solve?

Issue Number: close #xxx

What is changed and how it works?

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.
  • Removed unused message queue and sink metric definitions while preserving checkpoint delivery 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: 377f3ef4-2aa2-4af8-86c1-1d0fdc01e740

📥 Commits

Reviewing files that changed from the base of the PR and between c3c98e7 and 918306c.

📒 Files selected for processing (5)

• downstreamadapter/sink/cloudstorage/sink.go
• downstreamadapter/sink/kafka/sink.go
• downstreamadapter/sink/metrics/mq.go
• downstreamadapter/sink/pulsar/sink.go
• pkg/metrics/sink.go

💤 Files with no reviewable changes (3)

• downstreamadapter/sink/pulsar/sink.go
• downstreamadapter/sink/kafka/sink.go
• pkg/metrics/sink.go

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📝 Walkthrough

Walkthrough

The change removes checkpoint message metrics from cloud storage, Kafka, and Pulsar sinks. It also removes unused MQ metric definitions and registrations while preserving synchronized shared-registry storage and checkpoint delivery behavior.

Changes

Checkpoint 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. Encoding and delivery remain unchanged.

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

Possibly related PRs

• pingcap/ticdc#5801: Removes overlapping sink checkpoint and MQ metric definitions and registrations.
• pingcap/ticdc#5818: Changes logging in the Kafka sink file alongside this metric instrumentation cleanup.

Suggested reviewers: wk989898, hongyunyan

Poem

A rabbit found metrics in a checkpoint trail,
And trimmed unused counters from the sail.
Kafka, Pulsar, cloud storage align,
The registry stays locked and fine.
“Hop onward,” said Bun, “the sends still shine!”

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description retains an unfilled template and omits the problem, implementation details, valid issue reference, test evidence, impact assessment, and release note.	Complete each required section, replace the #xxx placeholder with a valid issue reference, and document tests, compatibility impact, documentation needs, and release notes.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title identifies the metrics fix, but it names Kafka although the changes also affect other sinks and shared metrics.
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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