code: public static Bson rename(final String fieldName, final String newFieldName) {
          notNull("newFieldName", newFieldName);
          return new SimpleUpdate<String>(fieldName, newFieldName, "$rename");
      }
docs: Creates an update that renames a field.
  
  @param fieldName    the non-null field name
  @param newFieldName the non-null new field name
  @return the update
  @mongodb.driver.manual reference/operator/update/rename/ $rename