move check if metrics are enabled to before action

app/controllers/metrics_controller.rb

@@ -3,6 +3,7 @@ require 'prometheus/client/formats/text'
 class MetricsController < ActionController::Base
   protect_from_forgery with: :exception
   include RequiresHealthToken
+  before_action :ensure_prometheus_metrics_are_enabled
 
   CHECKS = [
     Gitlab::HealthChecks::DbCheck,
@@ -10,9 +11,8 @@ class MetricsController < ActionController::Base
     Gitlab::HealthChecks::FsShardsCheck
   ].freeze
 
-  def metrics
-    return render_404 unless Gitlab::Metrics.prometheus_metrics_enabled?
 
+  def metrics
     metrics_text = Prometheus::Client::Formats::Text.marshal_multiprocess(multiprocess_metrics_path)
     response = health_metrics_text + "\n" + metrics_text
 
@@ -21,6 +21,10 @@ class MetricsController < ActionController::Base
 
   private
 
+  def ensure_prometheus_metrics_are_enabled
+    return render_404 unless Gitlab::Metrics.prometheus_metrics_enabled?
+  end
+
   def multiprocess_metrics_path
     Rails.root.join(ENV['prometheus_multiproc_dir'])
   end

spec/controllers/metrics_controller_spec.rb

@@ -48,6 +48,15 @@ describe MetricsController do
         expect(response.body).to match(/^filesystem_read_latency{shard="default"} [0-9\.]+$/)
         expect(response.body).to match(/^filesystem_readable{shard="default"} 1$/)
       end
+
+      context 'prometheus metrics are disabled' do
+        allow(Gitlab::Metrics).to receive(:prometheus_metrics_enabled?).and_return(false)
+
+        it 'returns proper response' do
+          get :metrics
+          expect(response.status).to eq(404)
+        end
+      end
     end
 
     context 'without authorization token' do
@@ -56,5 +65,6 @@ describe MetricsController do
         expect(response.status).to eq(404)
       end
     end
+
   end
 end