Remove deprecated Prometheus metrics from the docs

Our documentation GitLab Prometheus metrics lists a number of metrics, but some of them are not available anymore. Few examples:

  • redis_ping_latency_seconds
  • db_ping_latency_seconds
  • filesystem_access_latency_seconds
  • ...

We need to go through the list available in the docs, remove all obsolete metrics and add new ones if any.

Edited by Alexandr Tanayno