Reimplement 'labels' as a derivative of the Metric instead of a closure

[jdmarshall]

With the existence of LabelMap, there are simpler mechanisms available to give multiple callers a set of labels to apply to all metrics calls.

Problem areas:

• copy constructor
• reset - I already have a TODO in the code about how reset is poorly implemented, and this will need to be fixed.
• metrics gathering - a derived metric has already had its stats reported and should return empty set

In the bargain however, this one feature addresses #777, #407, and #598 all without introducing performance regressions.