Use deb for prometheus fluentd plugin

Change-Id: Ifbeb36e3d66c744f71bc209889cc56ddf6ba6430
diff --git a/fluentd/label/default_metric/prometheus.yml b/fluentd/label/default_metric/prometheus.yml
index 9eb3b1b..a13b786 100644
--- a/fluentd/label/default_metric/prometheus.yml
+++ b/fluentd/label/default_metric/prometheus.yml
@@ -3,7 +3,7 @@
     agent:
       plugin:
         fluent-plugin-prometheus:
-          gem: ['fluent-plugin-prometheus']
+          deb: ['td-agent-additional-plugins']
       config:
         input:
           prometheus: