tutorial: collecting and graphing Zincati metrics

[lucab]

This came up once (implicitly) in coreos/zincati#407 and once again recently in a private chat with @kelvinfan001.

It is still not very clear how Zincati metrics should be consumed, i.e. how to collect them in Prometheus and graph them in Grafana.
Right now we have a low-level reference doc and Grafana mixins in the repository.

What we are really missing is a tutorial showing:

• the local unix-socket endpoint for metrics
• how to bridge the local endpoint to HTTP via https://github.com/lucab/local_exporter or similar
• how to configure Prometheus to scrape Zincati metrics bridged over HTTP
• how to configure the mixins in Grafana in order to get some useful dashboards out of Zincati metrics