Add prometheus metrics collection
Set up prometheus metrics collection for the application. Ideally we should expose a small HTTP endpoint where the metrics can be collected.
Within this issue track the count of incoming logical replication messages. Metadata:
- table name (relname)
- operation: insert/update/delete
Optional, needs some research: track the byte size of each message (https://github.com/jackc/pglogrepl/blob/master/message.go#L643)