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
FWIW, I think charm URLs serialize nicely -- but you are actually probably correct not to use a *charm.URL in the doc struct, because then at least we don't have the db schema *directly* depending on code defined elsewhere. (we do in other places, I know...)