Add pgbouncer_exporter to Statefulset

[KamalAman]

What is missing?
Currently only Postgres metrics are exported, but there are no metrics on pgbouncer. Add https://github.com/prometheus-community/pgbouncer_exporter as an optional container to the statefulset in the single cluster chart.

Why do we need it?
I would like to monitor the number of connections being made to the pgbouncer

Anything else we need to know?:
Thank you! <3

[ascknx]

timescale/timescaledb-docker-ha#408 -> looks like the pgbouncer_exporter was already added to the docker image