Extract EE-specific lines for IssueEntity

app/serializers/issue_entity.rb

@@ -2,6 +2,7 @@
 
 class IssueEntity < IssuableEntity
   include TimeTrackableEntity
+  prepend ::EE::IssueEntity
 
   expose :state
   expose :milestone_id
@@ -18,9 +19,6 @@ class IssueEntity < IssuableEntity
   expose :due_date
   expose :moved_to_id
   expose :project_id
-  expose :weight, if: ->(issue, _) { issue.supports_weight? }
-  expose :milestone, using: API::Entities::Milestone
-  expose :labels, using: LabelEntity
 
   expose :web_url do |issue|
     project_issue_path(issue.project, issue)

ee/app/serializers/ee/issue_entity.rb

+# frozen_string_literal: true
+module EE
+  module IssueEntity
+    extend ActiveSupport::Concern
+
+    prepended do
+      expose :weight, if: ->(issue, _) { issue.supports_weight? }
+    end
+  end
+end