The existing implementation of the `python_twisted_reactor_tick_time` metric is pretty useless, because it *only* 
measures the time taken to execute timed calls and callbacks from threads. That neglects everything that 
happens off the back of I/O, which is obviously quite a lot for us.

To improve this, I've hooked into a different place in the reactor - in particular, where it calls `epoll`. That call is 
the only place it should wait for something to happen - the rest of the loop *should* be quick.

I've also removed `python_twisted_reactor_pending_calls`, because I don't believe anyone ever looks at it, and
it's a nuisance to populate.

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>

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

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.