docs: def query_tag_values(self, metric_type=None, **tags):
          """
          Query for possible tag values.
  
          :param metric_type: A MetricType to be queried. If left to None, matches all the MetricTypes
          :param tags: A dict of tag key/value pairs. Uses Hawkular-Metrics tag query language for syntax
          """
          tagql = self._transform_tags(**tags)
  
          return self._get(self._get_metrics_tags_url(self._get_url(metric_type)) + '/{}'.format(tagql))