code: def get_metric_descriptor(self):
            # noqa
          with self._md_cache_lock:
              if self._metric_descriptor is None:
                  self._metric_descriptor = metric_descriptor.MetricDescriptor(
                      self.name,
                      self.description,
                      self.measure.unit,
                      metric_utils.get_metric_type(self.measure,
                                                   self.aggregation),
                      # TODO: add label key description
                      [label_key.LabelKey(tk, "") for tk in self.columns])
          return self._metric_descriptor
docs: Get a MetricDescriptor for this view.
  
          Lazily creates a MetricDescriptor for metrics conversion.
  
          :rtype: :class:
                  `opencensus.metrics.export.metric_descriptor.MetricDescriptor`
          :return: A converted Metric.