Add Prometheus and Grafana to Docker Compose (#221)

* Add Prometheus and Grafana to Docker Compose

* Expose relay's Prometheus metrics port

* Use Docker network references intead of localhost

When you have containers on the same network they don't communicate
over localhost, they instead refer to their container names

* Move dashboard components into deployment folder

* Update folder structure for Grafana and Prometheus config files

The new folder structure more closely matches the expected defaults
by Grafana and Prometheus, which allows us to clean up the paths
in our docker-compose file a bit.

* Add documentation about Prometheus and Grafana

* Refer to Prometheus server instead of node