dailyののrotation世代管理

      def rotation(file_path, max_history)
        if max_history.nil?
          return
        end

        allow_filenames = 1.upto(max_history).to_a.map { |i|
          date = Date.today.prev_day(i)
          "#{file_path}.#{date.strftime('%Y%m%d')}"
        }
        allow_filenames.unshift(file_path)
        Dir.glob("#{File.dirname(file_path)}/*").each do |target_file_path|
          unless allow_filenames.include?(target_file_path)
            File.delete(target_file_path)
          end
        end

      end