fluent-plugin-inline-classifier で reqtime を分類する ref: http://qiita.com/yosisa/items/01be2d2e74858aae440e

file0.txt

$ gem install fluent-plugin-inline-classifier

file1.txt

<match raw.access.**>
  type inline_classifier
  add_prefix classified
  remove_prefix raw
  <rule>
    key reqtime
    type range
    store speed
    class1 ~20ms * 0.02
    class2 20-50ms 0.02 0.05
    class3 50-100ms 0.05 0.1
    class4 100-200ms 0.1 0.2
    class5 200-500ms 0.2 0.5
    class6 500-1000ms 0.5 1.0
    class7 1000ms~ 1.0 *
  </rule>
</match>