fix prometheus-client version

Author: tarokkk

fluentd-image/v1.9/Dockerfile

@@ -27,7 +27,7 @@ RUN apk update \
  && gem install async-http -v 0.50.0 \
  && gem install ext_monitor -v 0.1.2 \
  && gem install fluentd -v 1.9.2 \
- && gem install prometheus-client -v 2.0.0 \
+ && gem install prometheus-client -v 0.9.0 \
  && gem install bigdecimal -v 1.4.4 \
  && gem install \
          specific_install \