Change /api/v2/instance to be enabled without authentication when limited federation mode is enabled (#34576)

app/serializers/rest/instance_serializer.rb

@@ -49,7 +49,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
   def usage
     {
       users: {
-        active_month: object.active_user_count(4),
+        active_month: limited_federation? ? 0 : object.active_user_count(4),
       },
     }
   end
@@ -99,6 +99,8 @@ class REST::InstanceSerializer < ActiveModel::Serializer
       translation: {
         enabled: TranslationService.configured?,
       },
+
+      limited_federation: limited_federation?,
     }
   end
 
@@ -127,6 +129,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     markdown.render(Setting.closed_registrations_message) if Setting.closed_registrations_message.present?
   end
 
+  def limited_federation?
+    Rails.configuration.x.limited_federation_mode
+  end
+
   def markdown
     @markdown ||= Redcarpet::Markdown.new(Redcarpet::Render::HTML, no_images: true)
   end