Add support for a prometheus postgres exporter

This commit introduces a new role that downloads and installs the
prometheus community postgres exporter  https://github.com/prometheus-community/postgres_exporter.
A new credential is added to matrix_postgres_additional_databases that
allows the exporter access to the database to gather statistics.
A new dashboard was added to the grafana role, with some refactoring
to enable the dashboard only if the new role is enabled.
I've included some basic instructions for how to enable the role in
the Docs section.

In terms of testing, I've tested enabling the role, and disabling
it to make sure it cleans up the container and systemd role.

roles/matrix-grafana/tasks/setup.yml

@@ -64,7 +64,7 @@
     mode: 0440
     owner: "{{ matrix_user_username }}"
     group: "{{ matrix_user_groupname }}"
-  with_items: "{{ matrix_grafana_dashboard_download_urls }}"
+  with_items: "{{ matrix_grafana_dashboard_download_urls_all }}"
   when: matrix_grafana_enabled|bool
 
 - name: Ensure matrix-grafana.service installed