$ bundle

$ gem install fluent-plugin-genhashvalue

<filter foo.**>
  type genhashvalue
  keys type,descr
  hash_type md5    # md5/sha1/sha256/sha512/mur128
  base64_enc true
  base91_enc false
  set_key _hash
  separator _
  inc_time_as_key true
  inc_tag_as_key false
</filter>

root@112133c12ee3:~/fluentd# echo '{"type":"log","descr":"description..."}' | ./bin/fluent-cat foo.test
root@112133c12ee3:~/fluentd# echo '{"type":"log","descr":"description..."}' | ./bin/fluent-cat foo.test
root@112133c12ee3:~/fluentd# echo '{"type":"log","descr":"description..."}' | ./bin/fluent-cat foo.test

2016-10-23 15:06:05 +0000 foo.test: {"type":"log","descr":"description...","_hash":"/B3pc4NBk6Z9Ph89k+ZL4Q=="}
2016-10-23 15:06:22 +0000 foo.test: {"type":"log","descr":"description...","_hash":"IgB25wc3M0QJfk0KteYygQ=="}
2016-10-23 15:06:37 +0000 foo.test: {"type":"log","descr":"description...","_hash":"vvDF6eWyX5Sc01AVw8P6Cw=="}