Add a new ``monitor``.

        :param monitor_class: a :class:`.Monitor` class.
        :param monitor_name: a unique name for the monitor.
        :param kwargs: dictionary of key-valued parameters for the monitor.
        :return: the :class:`.Monitor` added.

def add_monitor(self, actor, monitor_name, **params):
        '''Add a new ``monitor``.

        :param monitor_class: a :class:`.Monitor` class.
        :param monitor_name: a unique name for the monitor.
        :param kwargs: dictionary of key-valued parameters for the monitor.
        :return: the :class:`.Monitor` added.
        '''
        if monitor_name in self.registered:
            raise KeyError('Monitor "%s" already available' % monitor_name)
        params.update(actor.actorparams())
        params['name'] = monitor_name
        params['kind'] = 'monitor'
        return actor.spawn(**params)