Support nested flieds in CSV format

[Horkyze]

Consider the following record:

 {
    "hostname":"buster",
    "priority":"6",
    "selinux_context":"unconfined\n",
    "pid":"4866",
    "image_name":"nginx",
    "container_name":"fluentd-docker_nginx_1",
    "container_id":"109e52fb8de8",
    "message":{
       "remote":"172.20.0.1",
       "host":"-",
       "user":"-",
       "method":"GET",
       "path":"/",
       "code":"200",
       "size":"612",
       "referer":"-",
       "agent":"curl/7.64.0",
       "http_x_forwarded_for":"-"
    },
    "source_realtime_timestamp":"1569258331312517"
 }

I would be nice to access the flieds in message fields in CSV format

  <filter **>
    @type parser
    key_name message
    reserve_data true
    hash_value_field message
    <parse>
      @type regexp
      expression /^(?<method>[^ ]*) (?<path>[^ ]*) (?<code>[0-9]*) (?<ms>[^ ]*) ms - (?<bytes_out>[0-9-]*)$/
    </parse>
  </filter>

  <match **>
    #@type file
    @type stdout

    #path /tmp/access.log
    <format>
      @type csv
      force_quotes false
      delimiter " "
      fields container_tag,message['method']
    </format>
  </match>

I have tried message['method'] message.method and message[:method] none of them works

[repeatedly]

I have tried message['method'] message.method and message[:method] none of them works

Because ruby's CSV module doesn't support nested field.
To support nested field, need to create target array before passing to CSV module.

[repeatedly]

Patch: #2643