Add a /metrics endpoint

[ballenspectric]

Some stats/metrics are currently displayed on the index page, but they are not formatted in a way that can be consumed by tools like Prometheus.

[ndmitch311]

@ballenspectric do you remember more specifics about which index page this refers to?

[ballenspectric]

It refers to the index page generated by this route, which has some stats and metrics on it:
https://github.com/spectriclabs/elastic_datashader/blob/master/elastic_datashader/routers/index.py
https://github.com/spectriclabs/elastic_datashader/blob/master/elastic_datashader/templates/index.html

Basically if you go to / or /index.html of the elastic_datashader deployment, it will display this page. I thought it might be useful to have these stats rendered in Prometheus format at a /metrics endpoint so it would be easier to monitor the heath of elastic_datashader from a common dashboard.