JSON Export Endpoint

[virtuallynathan]

Is your feature request related to a problem? Please describe. I'd like to use this exporter data elsewhere, and parsing the prometheus lines is not as easy as JSON

Describe the solution you'd like /metrics_json -> same data as JSON Potential solution: https://gist.github.com/ionutvilie/6ff915d19eaa3116b9b796463276c6dd

I can possibly do a PR, but since this is all docker-ized and integrated and such, It's not super straightforward.