Returns a dynamic MetricsHandler class tied
           to the passed registry.

def factory(cls, registry):
        """Returns a dynamic MetricsHandler class tied
           to the passed registry.
        """
        # This implementation relies on MetricsHandler.registry
        #  (defined above and defaulted to REGISTRY).

        # As we have unicode_literals, we need to create a str()
        #  object for type().
        cls_name = str(cls.__name__)
        MyMetricsHandler = type(cls_name, (cls, object),
                                {"registry": registry})
        return MyMetricsHandler