How-to doc for telemetry

## What is the problem your feature solves, or the need it fulfills?

Right now there's [user guide docs](https://github.com/cloudflare/pingora/blob/main/docs/user_guide/prom.md) on how to set up a Prometheus endpoint for metrics reporting. It should also be fairly easy to integrate certain other telemetry and observability frameworks like OpenTelemetry, though it would be nice to have a quick how-to guide to show this.

## Describe the solution you'd like

`user_guide` docs to provide more examples of what telemetry we support.