feat: enhance PostgreSQL Grafana Dashboard

This issue is to request 2 enhancements to the PostgreSQL dashboard provided via relation to COS:

## database stats (combined)

- currently the PostgreSQL database stats are shown per-unit. It would be great to have aggregated statistics across all units in a PostgreSQL deployment. So we can see e.g. the aggregated number of operations per second across all units.

## Remaining storage space on unit

- There is currently no indication in the PostgreSQL dashboard regarding remaining disk space on the unit.
- Data regarding remaining disk space is exposed via the general "System Resources" dashboard in COS, but this can be counter-intuitive to the user to search for. Also unclear if this is showing the correct remaining disk size (might be for the host itself and not the Juju storage where postgreSQL data is stored?)
- It would be best to have some indication on "remaining disk percentage / raw size in GB" inside the PostgreSQL dashboards themselves.

Start by locating the PostgreSQL Grafana dashboard provided through COS and inspect the existing per-unit database statistics and storage metrics. Done means the dashboard shows aggregated deployment-wide database statistics and reliable remaining storage as a percentage or raw size in GB for each unit.

Written by the indexing model from the issue text.