Added support for a metrics collections

Added a metric collection for storing unit metric in the state server

Only support for:

* adding new metrics
* getting lists of unsent metrics
* setting the sent flag
It'd be nice to use the MongoDB $currentDate, but that's only supported in 2.6. There's also a way to get the time out of a default _id, but we're using the UUID. I think it's probably ok to get the current time here.

Or, where does 'time' get set? Could we remove the param and set it to time.Now() in this method?