Generate a unique hash of every event to prevent duplicates

gistfile1.txt

# @see https://github.com/uken/fluent-plugin-elasticsearch#generate-hash-id
<filter **>
  @type elasticsearch_genid
  hash_id_key _hash
</filter>
# Apply value from filter to id_key field
# Elasticsearch also doesn't like field names with `_` so remove it.
<match **>
  @type elasticsearch
  hosts elasticsearch.local:9200
  [..]
  id_key _hash
  remove_keys _hash
  [..]
</match>