code: func RequireMetadataKeyUnchanged(key string) PreconditionFunc {
  	return func(patch interface{}) bool {
  		patchMap, ok := patch.(map[string]interface{})
  		if !ok {
  			return true
  		}
  		patchMap1, ok := patchMap["metadata"]
  		if !ok {
  			return true
  		}
  		patchMap2, ok := patchMap1.(map[string]interface{})
  		if !ok {
  			return true
  		}
  		_, ok = patchMap2[key]
  		return !ok
  	}
  }
docs: // RequireMetadataKeyUnchanged creates a precondition function that fails
  // if the metadata.key is present in the patch (indicating its value
  // has changed).