During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
Previously only Twisted's EPollReactor was compatible with the
reactor timing metric, notably not working when asyncio was used.
After this change, the following configurations support the reactor
timing metric:
* poll, epoll, or select reactors
* asyncio reactor with a poll, epoll, select, /dev/poll, or kqueue event loop.
Principally, `prometheus_client.REGISTRY.register` now requires its argument to
extend `prometheus_client.Collector`.
Additionally, `Gauge.set` is now annotated so that passing `Optional[int]`
causes an error.
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.
