<fields> does not handle nested records

[cosmo0920]

What would you like to be added:
Fluentd v1.0 or later provides record_accessor as a built-in core feature.
Currently, nested records are not able to become a dimensions.

record_accessor usage is:

• Include record_accessor module with: helpers :record_accessor
• Prepare record_accessor module's instances with: record_accessor_create(key)
• Using record_accessor module's methods; #call, #delete, #set

Why is this needed:
Sometimes, Fluentd handles logs that contain nested records. We'd appreciated if this feature could be provided in the future.
There is a workaround for this: Using filter_record_modifier to flatten nested record before processing and sending logs with this plugin.