contrib/gorilla/mux: skip tracing for some requests

[stroem]

Would be nice to skip creating spans for some of the requests.
Like e.g health-endpoints.

ochttp solves it this way (https://github.com/census-instrumentation/opencensus-go/blob/master/plugin/ochttp/server.go#L74):

svr = &http.Server{
    Addr: ":8080",
    Handler: &ochttp.Handler{
	    Handler:          s.mux,
	    IsPublicEndpoint: true,
	    IsHealthEndpoint: func(req *http.Request) bool {
		    switch req.URL.Path {
		    case "/health", "/health/":
			    return true
		    default:
			    return false
		    }
	    },
    },
  }

[gbbr]

This sounds good to me, and the PR is close to what we want. I'd like to suggest playing with the naming a bit, perhaps bringing it closer to what we have in the gRPC integration for similar functionality. How about WithIgnoreRequest ?