r/postgres-exporter: Deploy postgres-exporter

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/

postgresql.yml

@@ -9,3 +9,5 @@
     tags:
     - wal-g
   - postgresql-server
+  - role: postgres-exporter
+    tags: postgres-exporter