public Builder flattener(Flattener2 flattener) {
      this.flattener = flattener;
      return this;
    }
Set the flattener when print a log.

@param flattener the flattener when print a log
@return the builder
@since 1.6.0
xlog/src/main/java/com/elvishew/xlog/printer/file/FilePrinter.java