instrument OpenTracing with go-restful web framework

go-restful（https://github.com/emicklei/go-restful）is a widely-used library for building REST-style Web Services using Google Go. With powerful built-in modules including intelligent request routing, RESTful support and filters for intercepting HTTP request, go-restful make it very convenient to build a web application from scratch. This proposal aims at instrumenting OpenTracing with go-restful.

With OpenTracing instrumentation, go-restful web app developers can add (or switch) tracing implementations with an O(1) configuration change. More important, because OpenTracing is instrumented at go-restful library, a low enough level in the software stack, even large web application could be traced without additional annotations, application programmers also does not need to be aware of the tracing system.