Add proper stage.json data

app/controllers/projects/pipelines_controller.rb

@@ -104,9 +104,9 @@ class Projects::PipelinesController < Projects::ApplicationController
     @stage = pipeline.legacy_stage(params[:stage])
     return not_found unless @stage
-    respond_to do |format|
-      format.json { render json: { html: view_to_html_string('projects/pipelines/_stage') } }
-    end
+    render json: StageSerializer
+      .new(project: @project, current_user: @current_user)
+      .represent(@pipeline)
   end
   def retry

app/serializers/stage_details_entity.rb

+class StageDetailsEntity < Grape::Entity
+  include RequestAwareEntity
+
+  expose :name
+
+  expose :title do |stage|
+    "#{stage.name}: #{detailed_status.label}"
+  end
+
+  expose :statuses, with: JobEntity
+
+  expose :detailed_status, as: :status, with: StatusEntity
+
+  private
+
+  alias_method :stage, :object
+
+  def detailed_status
+    stage.detailed_status(request.current_user)
+  end
+end

app/serializers/stage_serializer.rb

+class StageSerializer < BaseSerializer
+  include WithPagination
+
+  InvalidResourceError = Class.new(StandardError)
+
+  entity StageDetailsEntity
+end