[codex] add telemetry events

[ruslandoga]

Summary

Adds first-class telemetry for query timing and connection lifecycle visibility.

• emits [:ch, :query, :start | :stop | :error]
• accumulates encode_time, queue_time, query_time, decode_time, and reused-connection idle_time into one final query event
• emits short connection events under [:ch, :conn, ...] for open, reuse, drop, and error behavior
• supports caller-provided :telemetry_metadata
• keeps query text and params out of metadata
• documents the event surface and adds success, query-error, connection-error, reuse, and drop coverage

Validation

• MIX_ENV=test mix compile --warnings-as-errors
• MIX_ENV=test mix format --check-formatted
• MIX_ENV=test mix deps.get --check-locked
• MIX_ENV=test mix deps.unlock --check-unused
• MIX_ENV=test mix test --include slow (481 passed)
• MIX_ENV=test mix coveralls.json --include slow (98.095%)
• MIX_ENV=test mix dialyzer --format github
• typos