fluentd output json value as json without message_key - fluentd

Question

I'm creating a log pipeline with filtering, transformations and multiple output routes.
I have a problem with outputing the raw log (without the "message_key").
Currently, the log looks like:
{"log": {"type": "debug", "log" :"This is the log message" , <More Entries>} }
I would like to drop the "log" message_key and output:
{"type": "debug", "log" :"This is the log message", <More Entries>}
I've tried:
1.
<filter *>
#type parser
key_name log
<parse>
#type json
</parse>
</filter>
And got an error probally since the the type is already a json.
2.
<filter *>
#type parser
key_name log
<parse>
#type none
</parse>
</filter>
And got this output (message "message_key" instead of the current "log"):
{"message": {"type": "debug", "log" :"This is the log message"} }
Tried to use the #type record_transformer, but the <record> want's a key-value and I would like to select the value only.
Tried to format under with single value, but the output was:
{"type" => "debug", "log" => "This is the log message"}
How can this be done? What's the best way to drop the message_key before outputing the log?

Answer 1

After skimming through the fluentd plugins here I didn't find a way to do what I wanted, so I've ended out writing my own plugin.
I'm not going to accept my answer since I hope someone will provide a better using a certified plugin.
Just in case you are desperate for a solution, here's the plugin:
require "fluent/plugin/filter"
module Fluent
module Plugin
class JsonRecordByKeyFilter < Fluent::Plugin::Filter
Fluent::Plugin.register_filter("json_record_by_key", self)
config_param :key
def filter(tag, time, record)
record[#key]
end
end
end
end
Usage:
<filter *>
#type json_record_by_key
key log
</filter>

Answer 2

Ran into this same problem, but found a solution as outlined on Fluentd's website using the remove_key_name_field command:
https://docs.fluentd.org/filter/parser#remove_key_name_field
<filter *>
#type parser
key_name log
reserve_data true
remove_key_name_field true
<parse>
#type json
</parse>
</filter>