Units for metric values

## The problem
How do we want to capture the units of the metric values that are output. I've seen different approaches from implicit assumptions, to adding into the dimension definitions or attributes and into the name of the dimensions.

We want to be consistent across the packages both in how we declare the values/units as well as the value of the unit itself. I don't think there is a standard CMEC way of doing this as far as I know @minxu74

A single diagnostic may produce many values of different units so it can't be a one-to-one definition on a `Metric`.

These units could be tracked in the DB in the metric value table or via a different data structure that is alongside or part of the CV. I've found it somewhat difficult to reason about within the CV. Is there one particular facet/dimension that defines the unit mapping (metric/statistic?)