Adds the `hidden` field to cycle analytics stages

ee/app/serializers/analytics/cycle_analytics/stage_entity.rb

@@ -4,6 +4,7 @@ module Analytics
   module CycleAnalytics
     class StageEntity < Grape::Entity
       expose :title
+      expose :hidden
       expose :legend
       expose :description
       expose :id

ee/spec/fixtures/api/schemas/analytics/cycle_analytics/stage.json

@@ -5,6 +5,9 @@
     "title": { 
       "type": "string" 
     },
+    "hidden": {
+      "type": "boolean"
+    },
     "legend": { 
       "type": "string" 
     },