Add comments to the Prometheus recording rules to make it clear which set of rules you need for Grafana or Prometheus Console. (#13876)

Remove unused Prometheus recording rules from `synapse-v2.rules` and add comments describing where the rest are used. (#13756)

The official dashboard uses data from these rules, but they were never added to the synapse-v2.rules. They are mentioned in this issue: https://github.com/matrix-org/synapse/issues/7917#issuecomment-661330409

, but never got added to the rules.

Adding them results in all graphs in the "Event persist rate" section to function as intended.

Signed-off-by: Johanna Dorothea Reichmann <transcaffeine@finallycoffee.eu>

This restores the config that is usable for prometheus pre v2.0.0
The new config only works for Prometheus v2+

... from https://github.com/matrix-org/synapse-prometheus-config.