contrib/gofiber/fiber.v2: capture error from fiber handler

The error returned from the fiber handler was being ignored
by the middleware. It now correctly captures the error and
sets it on the span.

Fixes DataDog#978

contrib/gofiber/fiber.v2/fiber.go

@@ -64,7 +64,9 @@ func Middleware(opts ...Option) func(c *fiber.Ctx) error {
 		}
 		span.SetTag(ext.HTTPCode, strconv.Itoa(status))
 
-		if cfg.isStatusError(status) {
+		if err != nil {
+			span.SetTag(ext.Error, err)
+		} else if cfg.isStatusError(status) {
 			// mark 5xx server error
 			span.SetTag(ext.Error, fmt.Errorf("%d: %s", status, http.StatusText(status)))
 		}

contrib/gofiber/fiber.v2/fiber_test.go

@@ -94,7 +94,7 @@ func TestTrace200(t *testing.T) {
 	})
 }
 
-func TestError(t *testing.T) {
+func TestStatusError(t *testing.T) {
 	assert := assert.New(t)
 	mt := mocktracer.Start()
 	defer mt.Stop()
@@ -128,6 +128,36 @@ func TestError(t *testing.T) {
 	assert.Equal(wantErr, span.Tag(ext.Error).(error).Error())
 }
 
+func TestCustomError(t *testing.T) {
+	assert := assert.New(t)
+	mt := mocktracer.Start()
+	defer mt.Stop()
+
+	router := fiber.New()
+	router.Use(Middleware(WithServiceName("foobar")))
+
+	router.Get("/err", func(c *fiber.Ctx) error {
+		c.SendStatus(400)
+		return fiber.ErrBadRequest
+	})
+	r := httptest.NewRequest("GET", "/err", nil)
+
+	response, err := router.Test(r, 100)
+	assert.Equal(nil, err)
+	assert.Equal(response.StatusCode, 400)
+
+	spans := mt.FinishedSpans()
+	assert.Len(spans, 1)
+	if len(spans) < 1 {
+		t.Fatalf("no spans")
+	}
+	span := spans[0]
+	assert.Equal("http.request", span.OperationName())
+	assert.Equal("foobar", span.Tag(ext.ServiceName))
+	assert.Equal("400", span.Tag(ext.HTTPCode))
+	assert.Equal(fiber.ErrBadRequest, span.Tag(ext.Error).(*fiber.Error))
+}
+
 func TestGetSpanNotInstrumented(t *testing.T) {
 	assert := assert.New(t)
 	router := fiber.New()