Document Prometheus metrics on bundle loads/failures

[sbernauer]

Takes less than 1 hour:

In #748 we added a bit more metrics.
This task is to quickly mention in the docs that users can build a Dashboard and Alerting on metrics such as bundle_failed_load_counter and bundle_loaded_counter.

Also link to the OPA docs https://www.openpolicyagent.org/docs/monitoring#status-metrics

[NickLarsenNZ]

A screenshot would also be nice, but optional.