During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
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.
Rather than hooking into the reactor loop, just add a timed task that runs every 100 ms to do the garbage collection.
Part 1 of a quest to simplify the reactor monkey-patching.
