Feature request: Add PGBouncer prometheus exporter

We're experimenting with a pgbouncer exporter in production but ultimately this should probably be in omnibus. So this is a feature request to give the Omnibus development team plenty of heads up to prioritise and schedule it.

The exporter we were looking at is https://github.com/larseen/pgbouncer_exporter/releases

The original issue for doing this in production was https://gitlab.com/gitlab-com/infrastructure/issues/2547 and the MR was gitlab-cookbooks/gitlab-prometheus!301 (closed)

However personally I think it actually makes more sense to just throw away the MR above and instead, once production is using the Omnibus HA setup, just download and run the exporter manually on staging and if we like the data from it go straight to putting it in Omnibus.

/cc @sitschner @pcarranza @ibaum @jtevnan