docs: Add auto-generated page for current metrics Prometheus exposes about itself.

Proposal

It would be amazing to have list of exact metric names and their help that Prometheus gives about itself. This is common for many products that expose Prometheus metrics, so would be nice to have Prometheus ones too (e.g head compactions). Perhaps we could expand on each HELP to tell more.

Extra: Have it autogenerate from source code would be amazing, so it's not requiring manual changes (or validation) on every PR.