docs: def add_metric(self, labels, count_value, sum_value, timestamp=None):
          """Add a metric to the metric family.
  
          Args:
            labels: A list of label values
            count_value: The count value of the metric.
            sum_value: The sum value of the metric.
          """
          self.samples.append(Sample(self.name + '_count', dict(zip(self._labelnames, labels)), count_value, timestamp))
          self.samples.append(Sample(self.name + '_sum', dict(zip(self._labelnames, labels)), sum_value, timestamp))