Produce prometheus metrics

Waveguide needs to produce prometheus metrics that our monitoring setup can consume. This will help visibility into what Waveguide servers are doing, and how we need to improve them.

Need ideas on what exactly we'd want to produce from a telemetry perspective.

No files, tests, or entry points are named. Start by defining which Waveguide server behaviors the monitoring setup needs to observe and confirm the expected Prometheus output with maintainers. Done means the telemetry scope and metric requirements are agreed and implemented for the relevant server behavior.

Written by the indexing model from the issue text.