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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_metric_descriptor(self):
 """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. """
  # 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