feat(webapp): add emission fan-out metrics to the native realtime feed

[ericallam]

Summary

Adds two counters to the native realtime backend so we can see how much duplicate row serialization the change router does per batch. When a run changes it can match several held feeds at once (a run subscription plus one or more tag/list feeds), and today each matching feed serializes that run's wire value independently. These counters quantify that fan-out so we can decide whether a shared serialization step is worth it.

What they measure

• realtime_native.emission_run_serializations: total wire-value serializations performed across feeds per batch (what the current path does).
• realtime_native.emission_distinct_serializations: distinct (columns, run) rows those serializations cover (what a serialize-once-per-batch step would do).

Average feeds-per-run is run_serializations / distinct_serializations, and 1 - distinct / run_serializations is the serialization work a shared step could save. Wired through a new optional onEmissionFanout callback on the router. No behavior change.

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Latest commit: 2b5e753

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[coderabbitai]

Walkthrough

Adds per-batch realtime emission fanout statistics to EnvChangeRouter, including total feed row deliveries and distinct (columnSig, runId) serializations. The native realtime client exposes Prometheus counters and records these values through the router callback. A changelog entry documents the new metrics.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description misses required template sections like Closes #, checklist, testing, changelog, and screenshots.	Add the missing template sections, including issue link, checklist items, testing steps, changelog entry, and screenshots or N/A.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly summarizes the main change: adding emission fan-out metrics to the native realtime feed.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/realtime-serialize-hotpath

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.