[P3] Add PR-review guardrail for new Sentry span instrumentation

Parent Epic: #43410 - Sentry Quota Breach Incident — Extension Telemetry
Labels: team-extension-platform
Priority: P3

🎯 Goal

Add a PR-review guardrail (checklist and/or CI lint) that catches quota-risky Sentry span instrumentation before it ships.

📝 Context

The 13.32.0 incident was caused by new custom spans (AssetsDataSourceTiming, AssetsUpdatePipeline) that fan out per trace, shipped with no per-trace sample gate and no kill-switch. Once released, the only remediation paths were a full release inbound filter (total data loss for the release) or forward-only code fixes. Catching the pattern at review time is the cheapest point of intervention.

✅ Acceptance Criteria

• Review checklist for PRs adding trace()/custom spans: fan-out analysis (spans per trace, traces per session), deterministic sub-sample gate, and a runtime kill-switch (env or remote flag).
• Evaluate a CI lint or codeowners rule on the TraceName registry / Sentry instrumentation paths so new instrumentation requires platform-team review.
• Guidance linked from the contributor docs and the telemetry-quota runbook.

Start by locating the TraceName registry, Sentry instrumentation paths, contributor docs, and telemetry-quota runbook mentioned in the acceptance criteria. Define the review checklist and assess a CI lint or codeowners rule for new trace() or custom-span instrumentation; done means the guidance is linked and new instrumentation receives the required quota, sampling, kill-switch, and platform-review checks.