code: func MetaNamespaceIndexFunc(obj interface{}) ([]string, error) {
  	meta, err := meta.Accessor(obj)
  	if err != nil {
  		return []string{""}, fmt.Errorf("object has no meta: %v", err)
  	}
  	return []string{meta.GetNamespace()}, nil
  }
docs: // MetaNamespaceIndexFunc is a default index function that indexes based on an object's namespace