Add a metric to the metric family.

        Args:
          labels: A list of label values
          value: A dict of labels

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

        Args:
          labels: A list of label values
          value: A dict of labels
        """
        self.samples.append(Sample(
            self.name + '_info',
            dict(dict(zip(self._labelnames, labels)), **value),
            1,
            timestamp,
        ))