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.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/683

Also includes the dashboards for Synapse and for Node Exporter.

Again has only been tested on debian amd64 so far, but the grafana docker image is available for arm64 and arm32. Nice.

Basic system stats, to show stuff the synapse metrics
can't show such as resource usage by bridges, etc

Seems to work fine as well.

This too has only been tested on debian amd64 so far

Initial attempt. Seems to work fine.

Only tested on debian amd64 so far

If we load it at runtime, during matrix-synapse role execution,
it's good enough for matrix-synapse and all roles after that,
but.. it breaks when someone uses `--tags=setup-nginx-proxy` alone.

The downside of including this vars file like this in `setup.yml`
is that the variables contained in it cannot be overriden by the user
(in their inventory's `vars.yml`).
... but it's not like overriding these variables was possible anyway
when including them at runtime.

Fixes #562 (Github Issue)

Fixes #586 (Github Issue)

Signed-off-by: Tulir Asokan <tulir@maunium.net>