fix nil pointer error in roundtripper

proxy/internal/metrics/metrics.go

@@ -106,15 +106,21 @@ func (f roundTripperFunc) RoundTrip(r *http.Request) (*http.Response, error) {
 
 func (m *Metrics) RoundTripper(next http.RoundTripper) http.RoundTripper {
 	return roundTripperFunc(func(req *http.Request) (*http.Response, error) {
-		start := time.Now()
-		res, err := next.RoundTrip(req)
-		duration := time.Since(start)
-
 		labels := prometheus.Labels{
 			"method": req.Method,
 			"host":   req.Host,
-			"path":   req.URL.Path,
+			// Fill potentially empty labels with default values to avoid cardinality issues.
+			"path":   "/",
+			"status": "0",
+			"size":   "0",
 		}
+		if req.URL != nil {
+			labels["path"] = req.URL.Path
+		}
+
+		start := time.Now()
+		res, err := next.RoundTrip(req)
+		duration := time.Since(start)
 
 		// Not all labels will be available if there was an error.
 		if res != nil {

proxy/internal/metrics/metrics_test.go

@@ -0,0 +1,64 @@
+package metrics_test
+
+import (
+	"net/http"
+	"net/url"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/netbirdio/netbird/proxy/internal/metrics"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type testRoundTripper struct {
+	response *http.Response
+	err      error
+}
+
+func (t *testRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	return t.response, t.err
+}
+
+func TestMetrics_RoundTripper(t *testing.T) {
+	testResponse := http.Response{
+		StatusCode: http.StatusOK,
+		Body:       http.NoBody,
+	}
+
+	tests := map[string]struct {
+		roundTripper http.RoundTripper
+		request      *http.Request
+		response     *http.Response
+		err          error
+	}{
+		"ok": {
+			roundTripper: &testRoundTripper{response: &testResponse},
+			request:      &http.Request{Method: "GET", URL: &url.URL{Path: "/foo"}},
+			response:     &testResponse,
+		},
+		"nil url": {
+			roundTripper: &testRoundTripper{response: &testResponse},
+			request:      &http.Request{Method: "GET", URL: nil},
+			response:     &testResponse,
+		},
+		"nil response": {
+			roundTripper: &testRoundTripper{response: nil},
+			request:      &http.Request{Method: "GET", URL: &url.URL{Path: "/foo"}},
+		},
+	}
+
+	m := metrics.New(prometheus.NewRegistry())
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			rt := m.RoundTripper(test.roundTripper)
+			res, err := rt.RoundTrip(test.request)
+			if diff := cmp.Diff(test.err, err); diff != "" {
+				t.Errorf("Incorrect error (-want +got):\n%s", diff)
+			}
+			if diff := cmp.Diff(test.response, res); diff != "" {
+				t.Errorf("Incorrect response (-want +got):\n%s", diff)
+			}
+		})
+	}
+}