Create & use an Experimentation::Shared module

Moves two constants from the Gitlab::Experimentation module into a new
Gitlab::Experimentation::Shared module which is then included everywhere
those two constants were previously used (and one additional place where
it might be nice to use them).

app/models/experiment.rb

 # frozen_string_literal: true
 
 class Experiment < ApplicationRecord
+  include ::Gitlab::Experimentation::Shared
+
   has_many :experiment_users
   has_many :users, through: :experiment_users
   has_many :control_group_users, -> { merge(ExperimentUser.control) }, through: :experiment_users, source: :user
@@ -14,7 +16,7 @@ class Experiment < ApplicationRecord
     return unless experiment
     return if experiment.experiment_users.where(user: user).exists?
 
-    group_type == ::Gitlab::Experimentation::GROUP_CONTROL ? experiment.add_control_user(user) : experiment.add_experimental_user(user)
+    group_type == GROUP_CONTROL ? experiment.add_control_user(user) : experiment.add_experimental_user(user)
   end
 
   def add_control_user(user)

app/models/experiment_user.rb

 # frozen_string_literal: true
 
 class ExperimentUser < ApplicationRecord
+  include ::Gitlab::Experimentation::Shared
+
   belongs_to :experiment
   belongs_to :user
 
-  enum group_type: { control: 0, experimental: 1 }
+  enum group_type: { GROUP_CONTROL => 0, GROUP_EXPERIMENTAL => 1 }
 
   validates :group_type, presence: true
 end

lib/gitlab/experimentation.rb

@@ -91,15 +91,13 @@ module Gitlab
       }
     }.freeze
 
-    GROUP_CONTROL = :control
-    GROUP_EXPERIMENTAL = :experimental
-
     # Controller concern that checks if an `experimentation_subject_id cookie` is present and sets it if absent.
     # Used for A/B testing of experimental features. Exposes the `experiment_enabled?(experiment_name)` method
     # to controllers and views. It returns true when the experiment is enabled and the user is selected as part
     # of the experimental group.
     #
     module ControllerConcern
+      include ::Gitlab::Experimentation::Shared
       extend ActiveSupport::Concern
 
       included do

lib/gitlab/experimentation/shared.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Experimentation
+    module Shared
+      GROUP_CONTROL = :control
+      GROUP_EXPERIMENTAL = :experimental
+    end
+  end
+end

spec/lib/gitlab/experimentation/shared_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Experimentation::Shared do
+  it 'defines a GROUP_CONTROL constant' do
+    expect(described_class.const_defined?(:GROUP_CONTROL)).to be_truthy
+  end
+
+  it 'defines a GROUP_EXPERIMENTAL constant' do
+    expect(described_class.const_defined?(:GROUP_EXPERIMENTAL)).to be_truthy
+  end
+end