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.

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))