An object of class Metric consists of a name, label, and a function
metric that takes arguments model (of class
Model) and out (of class Output), which
is the output of a method.
namea short name identifier. Must be alphanumeric.
labela longer, human readable label that can have other characters such as spaces, hyphens, etc.
metrica function with arguments "model" and "out" (and optionally "draw")
This class inherits from the Component class.