'Adds the specified labels to the message\'s labels set.  If a given
label is already present, it will not be duplicated.
Note that this function does not check against dryrun settings,
so you need to ensure that it is never called in a
dryrun mode.
:param labels: A set() of labels'

def addmessagelabels(self, uid, labels):

    newlabels = (self.getmessagelabels(uid) | labels)
    self.savemessagelabels(uid, newlabels)
