otelhttp: handle request Pattern

CHANGELOG.md

@@ -23,6 +23,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add `http.route` metric attribute in `go.opentelemetry.io/contrib/instrumentation/github.com/gin-gonic/gin/otelgin`. (#7275)
 - Add the `WithSpanStartOptions` option to add custom options to new spans `go.opentelemetry.io/contrib/instrumentation/github.com/gin-gonic/gin/otelgin`. (#7261)
 - Add instrumentation support for `go.mongodb.org/mongo-driver/v2` in `go.opentelemetry.io/contrib/instrumentation/go.mongodb.org/mongo-driver/v2/mongo/otelmongo`. (#6539)
+- Rerun the span name formatter after the request ran if a `req.Pattern` is set, so the span name can include it in `go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp`. (#7192)
 
 ### Changed
 

instrumentation/net/http/otelhttp/config.go

@@ -176,6 +176,10 @@ func WithMessageEvents(events ...event) Option {
 
 // WithSpanNameFormatter takes a function that will be called on every
 // request and the returned string will become the Span Name.
+//
+// When using [http.ServeMux] (or any middleware that sets the Pattern of [http.Request]),
+// the span name formatter will run twice. Once when the span is created, and
+// second time after the middleware, so the pattern can be used.
 func WithSpanNameFormatter(f func(operation string, r *http.Request) string) Option {
 	return optionFunc(func(c *config) {
 		c.SpanNameFormatter = f

instrumentation/net/http/otelhttp/handler.go

@@ -176,7 +176,12 @@ func (h *middleware) serveHTTP(w http.ResponseWriter, r *http.Request, next http
 		ctx = ContextWithLabeler(ctx, labeler)
 	}
 
-	next.ServeHTTP(w, r.WithContext(ctx))
+	r = r.WithContext(ctx)
+	next.ServeHTTP(w, r)
+
+	if r.Pattern != "" {
+		span.SetName(h.spanNameFormatter(h.operation, r))
+	}
 
 	statusCode := rww.StatusCode()
 	bytesWritten := rww.BytesWritten()

instrumentation/net/http/otelhttp/handler_test.go

@@ -437,6 +437,66 @@ func TestHandlerRequestWithTraceContext(t *testing.T) {
 	assert.Equal(t, spans[1].SpanContext().SpanID(), spans[0].Parent().SpanID())
 }
 
+func TestWithSpanNameFormatter(t *testing.T) {
+	for _, tt := range []struct {
+		name string
+
+		formatter    func(operation string, r *http.Request) string
+		wantSpanName string
+	}{
+		{
+			name:         "with the default span name formatter",
+			wantSpanName: "test_handler",
+		},
+		{
+			name: "with a custom span name formatter",
+			formatter: func(op string, r *http.Request) string {
+				return fmt.Sprintf("%s %s", r.Method, r.URL.Path)
+			},
+			wantSpanName: "GET /foo/123",
+		},
+		{
+			name: "with a custom span name formatter using the pattern",
+			formatter: func(op string, r *http.Request) string {
+				return fmt.Sprintf("%s %s", r.Method, r.Pattern)
+			},
+			wantSpanName: "GET /foo/{id}",
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			spanRecorder := tracetest.NewSpanRecorder()
+			provider := sdktrace.NewTracerProvider(
+				sdktrace.WithSpanProcessor(spanRecorder),
+			)
+
+			opts := []Option{
+				WithTracerProvider(provider),
+			}
+			if tt.formatter != nil {
+				opts = append(opts, WithSpanNameFormatter(tt.formatter))
+			}
+
+			mux := http.NewServeMux()
+			mux.Handle("/foo/{id}", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				// Nothing to do here
+			}))
+			h := NewHandler(mux, "test_handler", opts...)
+
+			r, err := http.NewRequest(http.MethodGet, "http://localhost/foo/123", nil)
+			require.NoError(t, err)
+
+			rr := httptest.NewRecorder()
+			h.ServeHTTP(rr, r)
+			assert.Equal(t, http.StatusOK, rr.Result().StatusCode)
+
+			assert.NoError(t, spanRecorder.ForceFlush(context.Background()))
+			spans := spanRecorder.Ended()
+			assert.Len(t, spans, 1)
+			assert.Equal(t, tt.wantSpanName, spans[0].Name())
+		})
+	}
+}
+
 func TestWithPublicEndpoint(t *testing.T) {
 	spanRecorder := tracetest.NewSpanRecorder()
 	provider := sdktrace.NewTracerProvider(