Add node monitoring to compose stack

[ulope]

Problem Description

We currently only expose monitoring of Synapse and the Traefik proxy.
To get better insight into the health of the systems we also need to expose node metrics via prometheus' node_exporter.

Tasks

• Add node_exporter to the docker-compose definition
• Expose node_exporter metrics on a sub-path of the metrics.<server-name> vhost

[fredo]

can be closed due to duplicate #200