Dm 1799 new metrics system mdj tweaks

Created by: matthew-d-jones

Functionality should be identical to Jonas's implementation in DM-1799_add_new_metrics, this is just refactored into a slightly different design. Processor is replaced by Reporters, one per different metrics Sink (logger, carbon etc). Interfacing with a different database, for example InfluxDB, would simply require implementing a new Sink and tweaking the Reporter factory function. Each Reporter is independent, so different Sinks could be configurable via the optargs.

Not ready for merging. The draft PR is up to invite feedback on the design.