dstat -> growthforecast にするにあたり map プラグイン使いたくなかったのでフィルタっぽいのを書く．out_exec_filter でやる方がいいかもしれんけど．

out_dstat_format.rb

# coding: utf-8
# dstat プラグインの出力を growthforecast 向けにデータ整形する
#
# 設定例:
# <match dstat>
#   type dstat_format
#   tag_format server.${hostname}.dstat
# </match>
#
class DstatFormatOutput < Fluent::Output
  Fluent::Plugin.register_output('dstat_format', self)

  config_param :tag_format, :default => 'dstat.${hostname}'
  config_param :key_format, :default => '${type}.${name}'
  # 好みで設定する
  config_param :keys, :default => "
    cpu.usr cpu.sys
    dsk.read dsk.writ
    net.recv net.send
    mem.used mem.buff mem.cach
  "
  config_param :additional_keys, :default => nil

  def configure(conf)
    super
    raw_keys = keys.split
    raw_keys += additional_keys.split if additional_keys
    @output_keys = raw_keys.map{|key| key.split(/\W+/) }
    @tag_name = @tag_format.gsub('${hostname}', `hostname`.chomp)
  end

  def key_name(type, name)
    @key_format.gsub('${type}', type).gsub('${name}', name)
  end

  def tag_name(hostname)
    @tag_format.gsub('${hostname}', hostname)
  end

  def get_stat_type(full_stat_name)
    # 必要に応じて修正
    case full_stat_name
    when /cpu usage/
      'cpu'
    else
      full_stat_name[0,3]
    end
  end

  def filter_dstat(dstat)
    dstat.each_with_object({}){|(k, v), result|
      stat_type = get_stat_type(k)
      v.each{|name, value|
        if @output_keys.include?([stat_type, name])
	  result[key_name(stat_type, name)] = value
        end
      }
    }
  end

  def emit(tag, es, chain)
    chain.next
    es.each {|time,record|
      filtered_dstat = filter_dstat(record["dstat"])
      Fluent::Engine.emit(tag_name(record["hostname"]), Fluent::Engine.now, filtered_dstat)
    }
  end
end