out_exec_filter で jq （内容は適当）

fluent.conf

<source>
  type forward
</source>

<match json.**>
  type exec_filter
  tag debug.json
  command jq '{a, b, c, x: .other.x, nextfoo: .other.next.foo, time}'
  in_format json
  out_format json
  time_key time
  flush_interval 1s
</match>

<match **>
  type stdout
</match>

sample.rb

require 'fluent-logger'
@logger = Fluent::Logger::FluentLogger.new('json')
def log
  @logger.post("foo", {
    a: rand(100), b: rand(100), c: rand(100),
    time: Time.now.to_i,
    other: {
      x: rand(10), y: rand(10), z: rand(10),
      next: { foo: rand(10) }
    }
  })
end
20.times do; puts Time.now; 10.times{ log }; sleep 0.5; end