Formatter Plugins: single_value how to get nested value

fluentd version: 1.12.2

record:
{ "content": { "log": "hello fluentd" } }

I only want get “log”. However, I can’t find how to get nested value.

I’m not sure, but it seems that it is enough to use record modifier.

It become something like this:

<filter ...>
  @type record_modifier
  whitelist_keys log
  <record>
    log ${record["content"]["log"]}
  </record>
</filter>