Make Prometheus metrics endpoint return empty response when metrics are disabled

app/controllers/metrics_controller.rb

@@ -3,10 +3,16 @@ class MetricsController < ActionController::Base
 
   protect_from_forgery with: :exception
 
-  before_action :validate_prometheus_metrics
-
   def index
-    render text: metrics_service.metrics_text, content_type: 'text/plain; version=0.0.4'
+    response = if Gitlab::Metrics.prometheus_metrics_enabled?
+                 metrics_service.metrics_text
+               else
+                 help_page = help_page_url('administration/monitoring/prometheus/gitlab_metrics',
+                                           anchor: 'gitlab-prometheus-metrics'
+                                          )
+                 "# Metrics are disabled, see: #{help_page}\n"
+               end
+    render text: response, content_type: 'text/plain; version=0.0.4'
   end
 
   private
@@ -14,8 +20,4 @@ class MetricsController < ActionController::Base
   def metrics_service
     @metrics_service ||= MetricsService.new
   end
-
-  def validate_prometheus_metrics
-    render_404 unless Gitlab::Metrics.prometheus_metrics_enabled?
-  end
 end

spec/controllers/metrics_controller_spec.rb

@@ -78,7 +78,8 @@ describe MetricsController do
         it 'returns proper response' do
           get :index
 
-          expect(response.status).to eq(404)
+          expect(response.status).to eq(200)
+          expect(response.body).to eq("# Metrics are disabled, see: http://test.host/help/administration/monitoring/prometheus/gitlab_metrics#gitlab-prometheus-metrics\n")
         end
       end
     end