docs: 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,
          ))