Slight refactoring of metered testing charms, specifying their names correctly.

In addition to that, charm loading was modified, so a charm with an empty metrics.yaml file will have a nil Metrics component (rather than an empty one).
Perhaps document for this function that if there are no metrics, a nil Metrics
value is returned. Otherwise it's pretty unintuitive - it's usual to assume that
if a nil error is returned, the other return value is valid. I'm not entirely convinced
that it's a great idea actually in this case - why do we want the charm author
to be able to provide an empty metrics.yaml file?