[feature request] Nested attributes

[koooge]

Hi there. ☀️ Parameter messege_keys seems not to be accepted nested value, right? I want to send the nested value such as fabric8io/fluent-plugin-kubernetes_metadata_filter plugin attaching nested output (like $.kubernetes.pod_name).

fluent-plugin-rewrite-tag-filter supports this https://github.com/fluent/fluent-plugin-rewrite-tag-filter#nested-attributes

btw record_accessor seems to require v0.14.20...

What do you think of this?

[rasheedamir]

@koooge it will be awesome! we also need exactly same thing; for now we just moved those attributes to root level to get things going