The [postgres-exporter][0] exposes PostgreSQL server statistics to
Prometheus. It connects to a specified PostgreSQL server (in this
case, a server on the local machine via UNIX socket) and collects data
from the `pg_stat_activity`, et al. views. It needs the `pg_monitor`
role in order to be allowed to read the relevant metrics.
Since we're setting up the exporter to connect via UNIX socket, it needs
a dedicated OS user to match the PostgreSQL user in order to
authenticate via the _peer_ method.
[0]: https://github.com/prometheus-community/postgres_exporter/
I am going to use the *postgresql* group for the dedicated database
servers. The configuration for those machines will be quite a bit
different than for the one existing machine that is a member of that
group already: the Nextcloud server. Rather than undefine/override all
the group-level settings at the host level, I have removed the Nextcloud
server from the *postgresql* group, and updated the `nextcloud.yml`
playbook to apply the *postgresql-server* role itself.
Eventually, I want to move the Nextcloud database to the central
database servers. At that point, I will remove the *postgresql-server*
role from the `nextcloud.yml` playbook.
Dustin