Use before action to respond with 404 if prometheus metrics are missing

app/controllers/projects/prometheus_controller.rb

 class Projects::PrometheusController < Projects::ApplicationController
   before_action :authorize_read_project!
+  before_action :require_prometheus_metrics!
 
   def active_metrics
-    return render_404 unless has_prometheus_metrics?
     matched_metrics = prometheus_service.reactive_query(Gitlab::Prometheus::Queries::MatchedMetricsQuery.name, &:itself)
 
     if matched_metrics
@@ -12,6 +12,8 @@ class Projects::PrometheusController < Projects::ApplicationController
     end
   end
 
+  private
+
   def prometheus_service
     project.monitoring_service
   end
@@ -19,4 +21,8 @@ class Projects::PrometheusController < Projects::ApplicationController
   def has_prometheus_metrics?
     prometheus_service&.respond_to?(:reactive_query)
   end
+
+  def require_prometheus_metrics!
+    render_404 unless has_prometheus_metrics?
+  end
 end