Added volumes for Prometheus and Grafana Compose services

[javabrett]

These volumes are per the recommended Docker Compose definitions for those containers. I think it's somewhat better to have these and allow restarts without data-loss, at the cost of needing to know to clean-up those volumes if you want a clean system - this is easily done by adding --volumes to docker-compose down.

This has helped me with demonstrations of the Metrics API so I don't lose test data if containers are restarted, and with development.

[Dabz]

It looks good, thanks for the contribution @javabrett , let me merge it ;)