code: @Override
      protected void trace(String msg, Object... args) {
          msg = S.fmt(msg, args);
          if (once) {
              msg = S.builder("[once]").append(msg).toString();
          }
          super.trace(msg);
      }
docs: Override parent implementation so that if this
  event bus is a one time event bus, it will prepend
  `[once]` into the message been logged
  
  @param msg
  the message
  @param args
  the message arguments