Check RawPath before checking Path on Prometheus middleware

Author: d6o

echoprometheus/prometheus.go

@@ -249,7 +249,15 @@ func (conf MiddlewareConfig) ToMiddleware() (echo.MiddlewareFunc, error) {
 			if url == "" {
 				// as of Echo v4.10.1 path is empty for 404 cases (when router did not find any matching routes)
 				// in this case we use actual path from request to have some distinction in Prometheus
-				url = c.Request().URL.Path
+
+				// Referencing Go documentation (https://cs.opensource.google/go/go/+/refs/tags/go1.21.3:src/net/url/url.go;l=357-359):
+				// We first check the RawPath, which is the original, escaped form. It's important to check RawPath first because
+				// it preserves the original encoding of the URL. Using Path (decoded form) can sometimes result invalid UTF-8 characters.
+				if c.Request().URL.RawPath != "" {
+					url = c.Request().URL.RawPath
+				} else {
+					url = c.Request().URL.Path
+				}
 			}
 
 			status := c.Response().Status

echoprometheus/prometheus_test.go

@@ -330,3 +330,22 @@ func unregisterDefaults(subsystem string) {
 		Help:      "The HTTP request sizes in bytes.",
 	})
 }
+
+func TestToMiddleware_RawPath(t *testing.T) {
+	middleware, err := MiddlewareConfig{}.ToMiddleware()
+	assert.NoError(t, err)
+
+	next := echo.HandlerFunc(func(c echo.Context) error {
+		return nil
+	})
+
+	handler := middleware(next)
+
+	e := echo.New()
+
+	req := httptest.NewRequest(http.MethodGet, "/v1/%a1", http.NoBody)
+	rec := httptest.NewRecorder()
+	echoContext := e.NewContext(req, rec)
+
+	assert.NoError(t, handler(echoContext))
+}