'allow aggregation of a custom stat'

def update_custom_stats(self, which, what, host=None):

    if (host is None):
        host = '_run'
    if ((host not in self.custom) or (which not in self.custom[host])):
        return self.set_custom_stats(which, what, host)
    if (not isinstance(what, type(self.custom[host][which]))):
        return None
    if isinstance(what, MutableMapping):
        self.custom[host][which] = merge_hash(self.custom[host][which], what)
    else:
        self.custom[host][which] += what
