High cpu usage with prometheus_monitor enabled

[ShunHsiangYou]

I have 2 nodes fluentd-aggregator cluster on AWS ECS. It's CPU is around 2% before I enable prometheus monitor. But whenever I enable prometheus monitor it becomes 40%

Dockerfile

FROM fluent/fluentd:v1.2.3-onbuild

RUN apk add --update --virtual .build-deps \
        sudo build-base ruby-dev \

 # cutomize following instruction as you wish
 && sudo gem install fluent-plugin-s3 \
 && sudo gem install fluent-plugin-flowcounter \
 && sudo gem install fluent-plugin-prometheus \

 && sudo gem sources --clear-all \
 && apk del .build-deps \
 && rm -rf /var/cache/apk/* \
           /home/fluent/.gem/ruby/2.3.0/cache/*.gem

COPY gems/fluent-plugin-dd-0.1.8.gem /home/fluent/

RUN gem install /home/fluent/fluent-plugin-dd-0.1.8.gem

COPY system.conf /fluentd/etc/
COPY sources.conf /fluentd/etc/

Configuration

fluentd.conf

@include system.conf
@include sources.conf

system.conf

<system>
  workers 4
</system>

source.conf

<source>
  @type forward
  port  24224
</source>

<source>
  @type debug_agent
  bind 127.0.0.1
  port 24230
</source>

<source>
  @type monitor_agent
  bind 0.0.0.0
  port 24220
</source>

<match fluent.**>
  @type stdout
</match>

# expose metrics in prometheus format
<source>
  @type prometheus
  bind 0.0.0.0
  port 24231
  metrics_path /metrics
</source>
<source>
  @type prometheus_output_monitor
  interval 10
  <labels>
    hostname ${hostname}
    worker ${worker_id}
  </labels>
</source>