Validate the excluded metrics. Returns the set of excluded params.

def _check_exclude(self, val):
        """
        Validate the excluded metrics. Returns the set of excluded params.
        """
        if val is None:
            exclude = frozenset()
        elif isinstance(val, str):
            exclude = frozenset([val.lower()])
        else:
            exclude = frozenset(map(lambda s: s.lower(), val))

        if len(exclude - frozenset(METRICS)) > 0:
            raise YellowbrickValueError(
                "'{}' is not a valid metric to exclude".format(repr(val))
            )

        return exclude