'Adds the activities to the feed on the given key
(The serialization is done by the serializer class)
:param key: the key at which the feed is stored
:param activities: the activities which to store'

def add_many(self, key, activities, *args, **kwargs):

    self.metrics.on_feed_write(self.__class__, len(activities))
    serialized_activities = self.serialize_activities(activities)
    return self.add_to_storage(key, serialized_activities, *args, **kwargs)
