code: public Map<String, OperationResult<ModelMetadataKey>> getChangedMetadataKeys(Map<String, ModelMetadataKey> keys) {
          String body = _serialize(keys);
          HttpResponse result = invoker.invoke("/metadatakey/changing" , body, HttpMethod.POST);
  
          if (result.getHttpCode() != HTTP_OK) {
              throw new ServiceException(ErrorCode.REMOTE_DIRECTORY_SERVER_ERROR,
                      "HTTP Code is not OK, code=%s", result.getHttpCode());
          }
  
          Map<String, OperationResult<ModelMetadataKey>> changedKeys = _deserialize(
                      result.getRetBody(), new TypeReference<Map<String, OperationResult<ModelMetadataKey>>>(){});
  
          return changedKeys;
      }
docs: Get the changed MetadataKey list.
  
  @param keys
  the MetadataKey List.
  @return
  the list of ModelMetadataKeys that have been changed.