At the moment, Rails::Rack::Logger tags the logger (if it's
ActiveSupport::TaggedLogging) for the duration of the @app.call, but
only fires the request.action_dispatch event later, on body close. That
means anything logged in request.action_dispatch handlers won't have the
same tags as the rest of the request.

Fix this by deferring the popping of tags into
finish_request_instrumentation, in the same way that finishing the
instrumentation handle is deferred.