Instead of a web.xml file, this uses a Servlet 3.1
ServletContainerInitializer to setup the app and tracing of it.
Initializer is indirectly invoked by SpringServletContainerInitializer,
which is in the classpath. This sets up the following:
- brave.example.Frontend and Backend : Rest controllers with no tracing configuration
- brave.example.TracingConfiguration : This adds tracing by configuring the tracer, server and client tracing interceptors.