Implement Prometheus Collector for Jug

[ddaws]

A Prometheus Collector collect stats on the MCD Jug contract. The Collect method of the collector will be called on every metrics collections from Prometheus. I propose naming metrics like mcd_jug_.... For all contract we can add metrics as mcd_<contract>_<metric>.

You can take a look at an example collector responsible for collecting metrics on Ethereum block headers here: https://github.com/ddaws/maker-monitor/blob/master/monitor/collector/header.go

Please use go-maker which supports accessing the Jug