code: private static boolean keyIsValid(final BinaryRequest request) {
          if (request.keyBytes() == null || request.keyBytes().length < MIN_KEY_BYTES) {
              request.observable().onError(new IllegalArgumentException("The Document ID must not be null or empty."));
              return false;
          }
  
          if (request.keyBytes().length > MAX_KEY_BYTES) {
              request.observable().onError(new IllegalArgumentException(
                  "The Document ID must not be longer than 250 bytes."));
              return false;
          }
  
          return true;
      }
docs: Helper method to check if the given request key is valid.
  
  If false is returned, the request observable is already failed.
  
  @param request the request to extract and validate the key from.
  @return true if valid, false otherwise.