code: func (s *ClusterConfigurationService) SetClusterName(c services.ClusterName) error {
  	value, err := services.GetClusterNameMarshaler().Marshal(c)
  	if err != nil {
  		return trace.Wrap(err)
  	}
  
  	_, err = s.Create(context.TODO(), backend.Item{
  		Key:     backend.Key(clusterConfigPrefix, namePrefix),
  		Value:   value,
  		Expires: c.Expiry(),
  	})
  	if err != nil {
  		return trace.Wrap(err)
  	}
  
  	return nil
  }
docs: // SetClusterName sets the name of the cluster in the backend. SetClusterName
  // can only be called once on a cluster after which it will return trace.AlreadyExists.