In Jetty 8 async servlets was on by default. In Jetty 9 it is off by default. and have to be added explicitly,customer use async servlets (continuations) to implement long polling.
BootstrapWebApplicationContextListener builds web.xml file from the one customer provide plus some extras like RequestStatisticsFilter.
RequestStatisticsFilter is added to the filter chain and it doesn't support async,and customer need a way to configure it.