message bus fixes and diagnostics

config/application.rb

@@ -20,6 +20,7 @@ module Discourse
     # -- all .rb files in that directory are automatically loaded.
 
     require 'discourse'
+    require 'message_bus_diags'
 
     # Custom directories with classes and modules you want to be autoloadable.
     config.autoload_paths += %W(#{config.root}/app/serializers)