Add default-level metrics and warn-level diagnostics to investigate
ServerTailnet reachability failures causing agent-specific 502s, without
restarting coderd or enabling debug logging.
- Enrich request logs and PTY/chatd warnings with agent ID, failure
reason, wait duration, coordinator state, WireGuard handshake and
byte counts, and DERP regions.
- Add metrics for unreachable attempts by reason, reachability latency,
embedded DERP connections, and offline app/terminal requests.
- Distinguish reachability failures from subsequent TCP dial failures,
cap reverse proxy dials at one minute, and exclude expected chatd
readiness polls and superseded-agent dials from unreachable counts.
- Report missing handshakes as zero time instead of 1970, also fixing
`coder ping`.
Part of PLAT-680, a sub-issue of PLAT-360.
Bobby Ho
603dd420d1