[mux] Add request filters like otelhttp (#4230)

CHANGELOG.md

@@ -32,6 +32,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - `WithRouteTag` in `go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp` adds HTTP route attribute to metrics. (#615)
 - Add `WithSpanOptions` option in `go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc`. (#3768)
 - Add testing support for Go 1.21. (#4233)
+- Add `WithFilter` option to `go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux`. (#4230)
 
 ### Changed
 

instrumentation/github.com/gorilla/mux/otelmux/config.go

@@ -28,6 +28,7 @@ type config struct {
 	spanNameFormatter func(string, *http.Request) string
 	PublicEndpoint    bool
 	PublicEndpointFn  func(*http.Request) bool
+	Filters           []Filter
 }
 
 // Option specifies instrumentation configuration options.
@@ -41,6 +42,10 @@ func (o optionFunc) apply(c *config) {
 	o(c)
 }
 
+// Filter is a predicate used to determine whether a given http.request should
+// be traced. A Filter must return true if the request should be traced.
+type Filter func(*http.Request) bool
+
 // WithPublicEndpoint configures the Handler to link the span with an incoming
 // span context. If this option is not provided, then the association is a child
 // association instead of a link.
@@ -91,3 +96,15 @@ func WithSpanNameFormatter(fn func(routeName string, r *http.Request) string) Op
 		cfg.spanNameFormatter = fn
 	})
 }
+
+// WithFilter adds a filter to the list of filters used by the handler.
+// If any filter indicates to exclude a request then the request will not be
+// traced. All filters must allow a request to be traced for a Span to be created.
+// If no filters are provided then all requests are traced.
+// Filters will be invoked for each processed request, it is advised to make them
+// simple and fast.
+func WithFilter(f Filter) Option {
+	return optionFunc(func(c *config) {
+		c.Filters = append(c.Filters, f)
+	})
+}

instrumentation/github.com/gorilla/mux/otelmux/mux.go

@@ -64,6 +64,7 @@ func Middleware(service string, opts ...Option) mux.MiddlewareFunc {
 			spanNameFormatter: cfg.spanNameFormatter,
 			publicEndpoint:    cfg.PublicEndpoint,
 			publicEndpointFn:  cfg.PublicEndpointFn,
+			filters:           cfg.Filters,
 		}
 	}
 }
@@ -76,6 +77,7 @@ type traceware struct {
 	spanNameFormatter func(string, *http.Request) string
 	publicEndpoint    bool
 	publicEndpointFn  func(*http.Request) bool
+	filters           []Filter
 }
 
 type recordingResponseWriter struct {
@@ -127,6 +129,14 @@ func defaultSpanNameFunc(routeName string, _ *http.Request) string { return rout
 // ServeHTTP implements the http.Handler interface. It does the actual
 // tracing of the request.
 func (tw traceware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	for _, f := range tw.filters {
+		if !f(r) {
+			// Simply pass through to the handler if a filter rejects the request
+			tw.handler.ServeHTTP(w, r)
+			return
+		}
+	}
+
 	ctx := tw.propagators.Extract(r.Context(), propagation.HeaderCarrier(r.Header))
 	routeStr := ""
 	route := mux.CurrentRoute(r)

instrumentation/github.com/gorilla/mux/otelmux/mux_test.go

@@ -162,3 +162,40 @@ func TestResponseWriterInterfaces(t *testing.T) {
 
 	router.ServeHTTP(w, r)
 }
+
+func TestFilter(t *testing.T) {
+	prop := propagation.TraceContext{}
+
+	router := mux.NewRouter()
+	var calledHealth, calledTest int
+	router.Use(Middleware("foobar", WithFilter(func(r *http.Request) bool {
+		return r.URL.Path != "/health"
+	})))
+	router.HandleFunc("/health", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calledHealth++
+		span := trace.SpanFromContext(r.Context())
+		assert.NotEqual(t, sc, span.SpanContext())
+		w.WriteHeader(http.StatusOK)
+	}))
+	router.HandleFunc("/test", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calledTest++
+		span := trace.SpanFromContext(r.Context())
+		assert.Equal(t, sc, span.SpanContext())
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	r := httptest.NewRequest("GET", "/health", nil)
+	ctx := trace.ContextWithRemoteSpanContext(context.Background(), sc)
+	prop.Inject(ctx, propagation.HeaderCarrier(r.Header))
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, r)
+
+	r = httptest.NewRequest("GET", "/test", nil)
+	ctx = trace.ContextWithRemoteSpanContext(context.Background(), sc)
+	prop.Inject(ctx, propagation.HeaderCarrier(r.Header))
+	w = httptest.NewRecorder()
+	router.ServeHTTP(w, r)
+
+	assert.Equal(t, 1, calledHealth, "failed to run test")
+	assert.Equal(t, 1, calledTest, "failed to run test")
+}