DEV: Deprecate show_in_ui argument of the register_stat API (#28727)

We need to start printing deprecation notices when the `show_in_ui` argument is used because it works only for the old about page which will be removed soon. For the new about page, we've introduced a new API `addAboutPageActivity` which is more flexible than a true/false argument on the server side.

Internal topic: t/136551.

lib/plugin/instance.rb

@@ -1122,9 +1122,24 @@ class Plugin::Instance
   # table. Some stats may be needed purely for reporting purposes and thus
   # do not need to be shown in the UI to admins/users.
   #
-  # TODO(osama): deprecate show_in_ui when experimental_redesigned_about_page_groups
-  # is removed
-  def register_stat(name, show_in_ui: false, expose_via_api: false, &block)
+  # TODO(osama): remove show_in_ui when experimental_redesigned_about_page_groups is removed
+  def register_stat(
+    name,
+    show_in_ui: (
+      not_using_deprecated_arg = true
+      false
+    ),
+    expose_via_api: false,
+    &block
+  )
+    if !not_using_deprecated_arg
+      Discourse.deprecate(
+        "`show_in_ui` argument of the `register_stat` API is deprecated. Please use the `addAboutPageActivity` JS API instead if you want your custom stat to be shown on the about page.",
+        since: "3.4.0.beta2",
+        drop_from: "3.5.0.beta1",
+      )
+    end
+
     # We do not want to register and display the same group multiple times.
     return if DiscoursePluginRegistry.stats.any? { |stat| stat.name == name }