Add a metric to the metric family.

        Args:
          labels: A list of label values
          buckets: A list of lists.
              Each inner list can be a pair of bucket name and value,
              or a triple of bucket name, value, and exemplar.
              The buckets must be sorted, and +Inf present.
          sum_value: The sum value of the metric.

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

        Args:
          labels: A list of label values
          buckets: A list of lists.
              Each inner list can be a pair of bucket name and value,
              or a triple of bucket name, value, and exemplar.
              The buckets must be sorted, and +Inf present.
          sum_value: The sum value of the metric.
        """
        for b in buckets:
            bucket, value = b[:2]
            exemplar = None
            if len(b) == 3:
                exemplar = b[2]
            self.samples.append(Sample(
                self.name + '_bucket',
                dict(list(zip(self._labelnames, labels)) + [('le', bucket)]),
                value,
                timestamp,
                exemplar,
            ))
        # +Inf is last and provides the count value.
        self.samples.extend([
            Sample(self.name + '_count', dict(zip(self._labelnames, labels)), buckets[-1][1], timestamp),
            Sample(self.name + '_sum', dict(zip(self._labelnames, labels)), sum_value, timestamp),
        ])