fixes#11950https://github.com/coder/coder/issues/11950#issuecomment-1987756088 explains the bug
We were also calling into `Unlisten()` and `Close()` while holding the mutex. I don't believe that `Close()` depends on the notification loop being unblocked, but it's hard to be sure, and the safest thing to do is assume it could block.
So, I added a unit test that fakes out `pq.Listener` and sends a bunch of notifies every time we call into it to hopefully prevent regression where we hold the mutex while calling into these functions.
It also removes the use of a `context.Context` to stop the PubSub -- it must be explicitly `Closed()`. This simplifies a bunch of the logic, and is how we use the pubsub anyway.
Adds prometheus metrics to PGPubsub for monitoring its health and performance in production.
Related to #11950 --- additional diagnostics to help figure out what's happening
