Improve `reactor_tick_time` metric (#11724)

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.