Implement stub_do_not_track helper

This helper can be used whenever there's a need to stub the DNT
request header.

The `TrackingHelpers` needs to be included in a spec for it to
work.

spec/controllers/concerns/redis_tracking_spec.rb

@@ -3,6 +3,8 @@
 require "spec_helper"
 
 RSpec.describe RedisTracking do
+  include TrackingHelpers
+
   let(:user) { create(:user) }
 
   controller(ApplicationController) do
@@ -60,7 +62,7 @@ RSpec.describe RedisTracking do
     end
 
     it 'tracks the event if DNT is not enabled' do
-      request.headers['DNT'] = '0'
+      stub_do_not_track('0')
 
       expect_tracking
 
@@ -68,7 +70,7 @@ RSpec.describe RedisTracking do
     end
 
     it 'does not track the event if DNT is enabled' do
-      request.headers['DNT'] = '1'
+      stub_do_not_track('1')
 
       expect_no_tracking
 

spec/controllers/projects/merge_requests/diffs_controller_spec.rb

@@ -4,6 +4,7 @@ require 'spec_helper'
 
 RSpec.describe Projects::MergeRequests::DiffsController do
   include ProjectForksHelper
+  include TrackingHelpers
 
   shared_examples '404 for unexistent diffable' do
     context 'when diffable does not exists' do
@@ -447,7 +448,7 @@ RSpec.describe Projects::MergeRequests::DiffsController do
 
       context 'when DNT is enabled' do
         before do
-          request.headers['DNT'] = '1'
+          stub_do_not_track('1')
         end
 
         it 'does not track any mr_diffs event' do

spec/lib/gitlab/experimentation/controller_concern_spec.rb

@@ -3,6 +3,8 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::Experimentation::ControllerConcern, type: :controller do
+  include TrackingHelpers
+
   before do
     stub_const('Gitlab::Experimentation::EXPERIMENTS', {
         backwards_compatible_test_experiment: {
@@ -43,7 +45,7 @@ RSpec.describe Gitlab::Experimentation::ControllerConcern, type: :controller do
     let(:cookie_value) { nil }
 
     before do
-      request.headers['DNT'] = do_not_track if do_not_track.present?
+      stub_do_not_track(do_not_track) if do_not_track.present?
       request.cookies[:experimentation_subject_id] = cookie_value if cookie_value
 
       get :index
@@ -242,7 +244,7 @@ RSpec.describe Gitlab::Experimentation::ControllerConcern, type: :controller do
 
       context 'do not track is disabled' do
         before do
-          request.headers['DNT'] = '0'
+          stub_do_not_track('0')
         end
 
         it 'does track the event' do
@@ -260,7 +262,7 @@ RSpec.describe Gitlab::Experimentation::ControllerConcern, type: :controller do
 
       context 'do not track enabled' do
         before do
-          request.headers['DNT'] = '1'
+          stub_do_not_track('1')
         end
 
         it 'does not track the event' do
@@ -396,7 +398,7 @@ RSpec.describe Gitlab::Experimentation::ControllerConcern, type: :controller do
 
       context 'do not track disabled' do
         before do
-          request.headers['DNT'] = '0'
+          stub_do_not_track('0')
         end
 
         it 'pushes the right parameters to gon' do
@@ -414,7 +416,7 @@ RSpec.describe Gitlab::Experimentation::ControllerConcern, type: :controller do
 
       context 'do not track enabled' do
         before do
-          request.headers['DNT'] = '1'
+          stub_do_not_track('1')
         end
 
         it 'does not push data to gon' do
@@ -525,7 +527,7 @@ RSpec.describe Gitlab::Experimentation::ControllerConcern, type: :controller do
 
       context 'is disabled' do
         before do
-          request.headers['DNT'] = '0'
+          stub_do_not_track('0')
           stub_experiment_for_subject(test_experiment: false)
         end
 
@@ -538,7 +540,7 @@ RSpec.describe Gitlab::Experimentation::ControllerConcern, type: :controller do
 
       context 'is enabled' do
         before do
-          request.headers['DNT'] = '1'
+          stub_do_not_track('1')
         end
 
         it 'does not call add_user on the Experiment model' do

spec/support/helpers/tracking_helpers.rb

+# frozen_string_literal: true
+
+module TrackingHelpers
+  def stub_do_not_track(value)
+    request.headers['DNT'] = value
+  end
+end

spec/support/shared_examples/controllers/unique_visits_shared_examples.rb

 # frozen_string_literal: true
 
 RSpec.shared_examples 'tracking unique visits' do |method|
+  include TrackingHelpers
+
   let(:request_params) { {} }
 
   it 'tracks unique visit if the format is HTML' do
@@ -14,14 +16,15 @@ RSpec.shared_examples 'tracking unique visits' do |method|
     expect(Gitlab::UsageDataCounters::HLLRedisCounter)
       .to receive(:track_event).with(target_id, values: kind_of(String))
 
-    request.headers['DNT'] = '0'
+    stub_do_not_track('0')
 
     get method, params: request_params, format: :html
   end
 
   it 'does not track unique visit if DNT is enabled' do
     expect(Gitlab::UsageDataCounters::HLLRedisCounter).not_to receive(:track_event)
-    request.headers['DNT'] = '1'
+
+    stub_do_not_track('1')
 
     get method, params: request_params, format: :html
   end