docs: def get_metadata(self, key, counter_id):
          """
          Return the metadata for the metric identified by `counter_id` for the given instance key.
          If the key is not in the cache, raises a KeyError.
          If there's no metric with the given counter_id, raises a MetadataNotFoundError.
          """
          with self._lock:
              metadata = self._metadata[key]
              try:
                  return metadata[counter_id]
              except KeyError:
                  raise MetadataNotFoundError("No metadata for counter id '{}' found in the cache.".format(counter_id))