Telemetry Essentials

Use this skill before writing ANY telemetry, logging, or metrics code.

RULES — Follow these with no exceptions

1. Use structured logging (Logger.info("action", key: value)) — never string interpolation in log messages
2. Attach telemetry handlers in Application.start/2 — not in modules that may restart, and never in GenServer init
3. Use Ecto.Repo telemetry events for query monitoring — Ecto already emits events; don't manually instrument queries
4. Use Phoenix.LiveDashboard in dev/staging — free observability with zero code
5. Tag telemetry events with metadata (user_id, request_id) — without correlation IDs, traces are useless
6. Never log at :debug level in production — it includes query parameters and PII; use :info level instead

Structured Logging