Logstash extract a nested json object

[imsystem]

Hello!
I have input json:

{
   "fields":{
      "created_date":1614158456696703782,
      "value":"\u001B[35m-\u001B[0m | \u001B[32m2021-02-20T12:48:10.367337+0300\u001B[0m | \u001B[1mINFO    \u001B[0m | application.app:<module>:98 - \u001B[1mtest info"
   },
   "@version":"1",
   "@timestamp":"2021-02-24T09:31:50.208Z",
   "tags":{
      "data_tags":"[\"tag1\",\"tag2\"]",
      "collection_platform":"telegraf",
      "index_name":"test_alias",
      "source_type":"file",
      "data_type":"raw",
      "path":"/opt/map/hub/logs/trace.log",
      "host":"hub"
   },
   "timestamp":1614158456,
   "name":"logparser"
}

How extract fields: created_date , value, if i dont know them?

If i use json plugin:

json {
  skip_on_invalid_json => false
  source => "fields"
}

or

mutate {
  convert => { "fields" => "string" }
}
json {
  skip_on_invalid_json => false
  source => "fields"
}

I have error:

Feb 24 12:21:24 elk logstash[2868]: [2021-02-24T12:21:24,951][WARN ][logstash.filters.json    ][main] Exception caught in json filter {:exception=>"class java.util.HashMap cannot be cast to class java.lang.String (java.util.HashMap and java.lang.String are in module java.base of loader 'bootstrap')", :source=>"fields", :raw=>{"created_date"=>1614158456696703782, "value"=>"\e[35m-\e[0m | \e[32m2021-02-20T12:48:10.367337+0300\e[0m | \e[1mINFO    \e[0m | application.app:<module>:98 - \e[1mtest info"}}
Feb 24 12:21:24 elk logstash[2868]: [2021-02-24T12:21:24,977][ERROR][org.logstash.execution.WorkerLoop][main] Exception in pipelineworker, the pipeline stopped processing new events, please check your filter configuration and restart Logstash.
Feb 24 12:21:24 elk logstash[2868]: java.lang.ClassCastException: class java.util.HashMap cannot be cast to class java.lang.String (java.util.HashMap and java.lang.String are in module java.base of loader 'bootstrap')

[Badger]

The error message shows that [fields] is an object, not a string (see here to understand why). You can move arbitrary fields within an object to the root using a ruby filter. See here for an example.

[imsystem]

Thanks!