WIP: Implemented the builtin metric unit-time

> unit-time will record the time a unit has a charmURL set on it.
> Recorded as a time on the unit. When converted to a metric it is represented as the number of seconds the unit has been running this charm
> A new api call is available to record builtin metrics and add them to the metrics collection in state
>> Most of the code in here is functions and tests for supporting this api call
> A new workers calls this api every 5 minutes