Add a metric to the metric family.

        Args:
          labels: A list of label values
          buckets: A list of pairs of bucket names and values.
              The buckets must be sorted, and +Inf present.
          gsum_value: The sum value of the metric.

def add_metric(self, labels, buckets, gsum_value, timestamp=None):
        """Add a metric to the metric family.

        Args:
          labels: A list of label values
          buckets: A list of pairs of bucket names and values.
              The buckets must be sorted, and +Inf present.
          gsum_value: The sum value of the metric.
        """
        for bucket, value in buckets:
            self.samples.append(Sample(
                self.name + '_bucket',
                dict(list(zip(self._labelnames, labels)) + [('le', bucket)]),
                value, timestamp))
        # +Inf is last and provides the count value.
        self.samples.extend([
            Sample(self.name + '_gcount', dict(zip(self._labelnames, labels)), buckets[-1][1], timestamp),
            Sample(self.name + '_gsum', dict(zip(self._labelnames, labels)), gsum_value, timestamp),
        ])