Add rescue_from(ActionController::UnknownFormat) in Application Controller

app/controllers/application_controller.rb

@@ -40,6 +40,10 @@ class ApplicationController < ActionController::Base
     render_404
   end
 
+  rescue_from(ActionController::UnknownFormat) do
+    render_404
+  end
+
   rescue_from Gitlab::Access::AccessDeniedError do |exception|
     render_403
   end

app/controllers/projects/deployments_controller.rb

@@ -25,12 +25,16 @@ class Projects::DeploymentsController < Projects::ApplicationController
   def additional_metrics
     return render_404 unless deployment.has_additional_metrics?
 
-    metrics = deployment.additional_metrics
+    respond_to do |format|
+      format.json do
+        metrics = deployment.additional_metrics
 
-    if metrics.any?
-      render json: metrics
-    else
-      head :no_content
+        if metrics.any?
+          render json: metrics
+        else
+          head :no_content
+        end
+      end
     end
   end
 

app/controllers/projects/environments_controller.rb

@@ -132,9 +132,13 @@ class Projects::EnvironmentsController < Projects::ApplicationController
   end
 
   def additional_metrics
-    additional_metrics = environment.additional_metrics || {}
+    respond_to do |format|
+      format.json do
+        additional_metrics = environment.additional_metrics || {}
 
-    render json: additional_metrics, status: additional_metrics.any? ? :ok : :no_content
+        render json: additional_metrics, status: additional_metrics.any? ? :ok : :no_content
+      end
+    end
   end
 
   private

app/controllers/projects/prometheus_controller.rb

@@ -18,10 +18,6 @@ class Projects::PrometheusController < Projects::ApplicationController
 
   private
 
-  rescue_from(ActionController::UnknownFormat) do
-    render_404
-  end
-
   def require_prometheus_metrics!
     render_404 unless project.prometheus_service.present?
   end

spec/controllers/application_controller_spec.rb

@@ -99,6 +99,36 @@ describe ApplicationController do
     end
   end
 
+  describe 'response format' do
+    controller(described_class) do
+      def index
+        respond_to do |format|
+          format.json do
+            head :ok
+          end
+        end
+      end
+    end
+
+    context 'when format is handled' do
+      let(:requested_format) { :json }
+
+      it 'returns 200 response' do
+        get :index, private_token: user.private_token, format: requested_format
+
+        expect(response).to have_http_status 200
+      end
+    end
+
+    context 'when format is not handled' do
+      it 'returns 404 response' do
+        get :index, private_token: user.private_token
+
+        expect(response).to have_http_status 404
+      end
+    end
+  end
+
   describe '#authenticate_user_from_rss_token' do
     describe "authenticating a user from an RSS token" do
       controller(described_class) do