docs: def ReadClientMetadata(self, client_id):
      """Reads the ClientMetadata record for a single client.
  
      Args:
        client_id: A GRR client id string, e.g. "C.ea3b2b71840d6fa7".
  
      Returns:
        An rdfvalues.object.ClientMetadata object.
  
      Raises:
        UnknownClientError: if no client with corresponding id was found.
      """
      result = self.MultiReadClientMetadata([client_id])
      try:
        return result[client_id]
      except KeyError:
        raise UnknownClientError(client_id)