Add missing prometheus rules for persisted events (#8802)

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>