Add a metric to the metric family.

        Args:
          labels: A list of label values
          value: The value of the metric
          created: Optional unix timestamp the child was created at.

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

        Args:
          labels: A list of label values
          value: The value of the metric
          created: Optional unix timestamp the child was created at.
        """
        self.samples.append(Sample(self.name + '_total', dict(zip(self._labelnames, labels)), value, timestamp))
        if created is not None:
            self.samples.append(Sample(self.name + '_created', dict(zip(self._labelnames, labels)), created, timestamp))