docs: def get_sample_value(self, name, labels=None):
          """Returns the sample value, or None if not found.
  
          This is inefficient, and intended only for use in unittests.
          """
          if labels is None:
              labels = {}
          for metric in self.collect():
              for s in metric.samples:
                  if s.name == name and s.labels == labels:
                      return s.value
          return None