[codex] add telemetry events

[ruslandoga]

Summary

Adds first-class telemetry instrumentation for Ch queries and connection lifecycle events.

• emits [:ch, :query, :start | :stop | :error] events with native-time phase measurements
• emits short connection lifecycle events under [:ch, :conn, ...] for open, reuse, drop, and error visibility
• supports :telemetry_metadata on queries for caller-provided context
• documents telemetry events and adds focused tests

Validation

• MIX_ENV=test mix test test/ch/telemetry_test.exs
• MIX_ENV=test mix test