the doc is in:
- https://www.elastic.co/guide/en/beats/filebeat/current/kafka-output.html
- https://www.elastic.co/guide/en/beats/filebeat/current/filebeat-reference-yml.html
- the filebeat-reference.yml in rpm
all of them are still recommend %{[type]}. but I test in filebeat 6.0.0, it can not refer to document_type
and thanks for 'topic: '%{[fields.log_topic]}'.' it work for me