code: public <T> String getChangeVectorFor(T instance) {
          if (instance == null) {
              throw new IllegalArgumentException("instance cannot be null");
          }
  
          DocumentInfo documentInfo = getDocumentInfo(instance);
          JsonNode changeVector = documentInfo.getMetadata().get(Constants.Documents.Metadata.CHANGE_VECTOR);
          if (changeVector != null) {
              return changeVector.asText();
          }
          return null;
      }
docs: Gets the Change Vector for the specified entity.
  If the entity is transient, it will load the change vector from the store
  and associate the current state of the entity with the change vector from the server.
  
  @param <T>      instance class
  @param instance Instance to get change vector from
  @return change vector