Monitoring is not working with Kafka2 output plugin

Hello,

Readme https://github.com/fluent/fluent-plugin-kafka/blob/master/README.md#output-plugin says that adding `monitoring_list` would enable monitoring based on `statsd` or `datadog` for `kafka2` output plugin. I have checked the `kafka2_out.rb` and seems like it's not included, but present in `kafka_buffered.rb`. I am using Fluentd v1, so how can I enable statsd monitoring for kafka2 output plugin?