docs: 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),
          ])