Add support for Grafana

[hatchan]

This allow am to automatically download and hook it up to the selected Prometheus instances.

It should also automatically setup the autometrics dashboard, and allow for a user to supply their own custom dashboard to be loaded automatically (this could possibly be a separate ticket).

[gagbo]

One thing to remember (just posting here before I forget) is to read/reuse the scrape_interval from the configured Prometheus to set it in the settings of the data source in Grafana. I have a strong suspicion that not configuring the interval can lead to empty dashboards like in autometrics-dev/autometrics-shared#66 for longer prometheus scrape intervals