code: public static String shortDebugString(final UnknownFieldSet fields) {
      try {
        final StringBuilder sb = new StringBuilder();
        SINGLE_LINE_PRINTER.printUnknownFields(fields, new TextGenerator(sb));
        // Single line mode currently might have an extra space at the end.
        return sb.toString().trim();
      } catch (IOException e) {
        throw new IllegalStateException(e);
      }
    }
docs: Generates a human readable form of the unknown fields, useful for debugging
  and other purposes, with no newline characters.