undefined method `to_msgpack' for 2019-12-17 23:45:44 +0530:Time - output plugin

Needs to store multiple Time type keys in mongodb. I am successfully able to change default time key to created_at but i want one more key with name updated_at.

>
@type record_transformer
enable_ruby

updated_at ${time}

>

@type copy

@type mongo
database test
host localhost:27017
user user1
password pwd1
collection logs
include_time_key true
time_key created_at