Rack tracing doesn't block static assets

As seen in this trace: https://herokuapp15263610.tv.appneta.com/traces/view/2E9AD7872EE16B9FCD209EA0DF74453447B9789F#

These generate really irrelevant traces that break averages. If you start a trace at the Rack layer, as per Heroku, it should probably block static assets by default like how our Apache/nginx/(soon Java) instrumentation does.