contrib/gorilla/mux: add option for skipping tracing (#789)

This update adds an option called WithSkippingFunc, which can be used for
skipping tracing based on request path.

Fixes #790

contrib/gorilla/mux/mux.go

@@ -94,6 +94,10 @@ func NewRouter(opts ...RouterOption) *Router {
 // We only need to rewrite this function to be able to trace
 // all the incoming requests to the underlying multiplexer
 func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	if r.config.ignoreRequest(req) {
+		r.Router.ServeHTTP(w, req)
+		return
+	}
 	var (
 		match    mux.RouteMatch
 		spanopts []ddtrace.StartSpanOption

contrib/gorilla/mux/mux_test.go

@@ -199,6 +199,40 @@ func TestAnalyticsSettings(t *testing.T) {
 	})
 }
 
+func TestIgnoreRequestOption(t *testing.T) {
+	tests := []struct {
+		url       string
+		spanCount int
+	}{
+		{
+			url:       "/skip",
+			spanCount: 0,
+		},
+		{
+			url:       "/200",
+			spanCount: 1,
+		},
+	}
+	mux := NewRouter(WithIgnoreRequest(func(req *http.Request) bool {
+		return req.URL.Path == "/skip"
+	}))
+	mux.Handle("/skip", okHandler()).Host("localhost")
+	mux.Handle("/200", okHandler()).Host("localhost")
+
+	for _, test := range tests {
+		t.Run(test.url, func(t *testing.T) {
+			mt := mocktracer.Start()
+			defer mt.Stop()
+			r := httptest.NewRequest("GET", "http://localhost"+test.url, nil)
+			w := httptest.NewRecorder()
+			mux.ServeHTTP(w, r)
+
+			spans := mt.FinishedSpans()
+			assert.Equal(t, test.spanCount, len(spans))
+		})
+	}
+}
+
 func TestResourceNamer(t *testing.T) {
 	staticName := "static resource name"
 	staticNamer := func(*Router, *http.Request) string {

contrib/gorilla/mux/option.go

@@ -21,6 +21,7 @@ type routerConfig struct {
 	finishOpts    []ddtrace.FinishOption    // span finish options to be applied
 	analyticsRate float64
 	resourceNamer func(*Router, *http.Request) string
+	ignoreRequest func(*http.Request) bool
 }
 
 // RouterOption represents an option that can be passed to NewRouter.
@@ -37,6 +38,15 @@ func defaults(cfg *routerConfig) {
 		cfg.serviceName = svc
 	}
 	cfg.resourceNamer = defaultResourceNamer
+	cfg.ignoreRequest = func(_ *http.Request) bool { return false }
+}
+
+// WithIgnoreRequest holds the function to use for determining if the
+// incoming HTTP request tracing should be skipped.
+func WithIgnoreRequest(f func(*http.Request) bool) RouterOption {
+	return func(cfg *routerConfig) {
+		cfg.ignoreRequest = f
+	}
 }
 
 // WithServiceName sets the given service name for the router.