feat(server): telemetry #55

The server logs with tracing to stderr and exports nothing. Running it for other people needs traces, metrics and logs somewhere they can be searched and alerted on.

Export them with OpenTelemetry when an endpoint is configured, and keep today's output when none is. The hot paths the benchmarks cover (requests, long-poll wakeups, pushes, fetches, landings) are the ones to measure. Tokens and file content never appear in telemetry.