Scripted Field for elements inside JSON objects

Hey Guys,
I'm using FluentD (deployed as DaemonSet) to stream k8s app (containers) logs to elasticsearch. i want to extract specific keys from the 'log' key string (see attached) in elastic such as logKey: ... or statusCode:..
i want to use scripted field ~ painless script.
i struggled a bit with it, can someone please advise?