Put all event metrics exposed to prometheus behind a feature flag

lib/gitlab/metrics/transaction.rb

@@ -155,6 +155,7 @@ module Gitlab
         fetch_metric(type, metric_name) do
           docstring "Transaction #{prefix}#{name} #{type}"
           base_labels tags.merge(BASE_LABELS)
+          with_feature "prometheus_transaction_#{prefix}#{name}_total".to_sym
 
           if type == :gauge
             multiprocess_mode :livesum