def get_history_tags(self, exp, rep=0):
        """ returns all available tags (logging keys) of the given experiment 
            repetition. 
            
            Note: Technically, each repetition could have different
            tags, therefore the rep number can be passed in as parameter, 
            even though usually all repetitions have the same tags. The default 
            repetition is 0 and in most cases, can be omitted.
        """
        history = self.get_history(exp, rep, 'all')
        return history.keys()