A handler that is able to tag requests. Usually mixed in to other handlers.
Instead of using the handler you should use Handler.Stage.
Handler.Stage is a handler improves upon the RequestTaggingHandler in several ways:
(a) Handler.Stage can be nested to arbitrary depth, (b) it doesn't require
mixing-in and (c) it allows handlers to be rewritten as well as requests, (d) it
prevents Play from accessing the real handler until its logic has been run.
(Since version 2.6.0) Use Handler.Stage instead