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
* chore: rename store to dbmock for consistency
* chore: remove redundant dbtype package
This wasn't necessary and forked how we do DB types.
* chore: separate pubsub into a new package
This didn't need to be in database and was bloating it.