lib/httpserver: properly quote the returned address from GetQuotedRemoteAddr() for requests with X-Forwarded-For header

Make sure that the quoted address can be used as JSON string.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/4676#issuecomment-1663203424

This is a follow up for 252643d100 and ac0b7e0421

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/4676

lib/httpserver/httpserver_test.go

@@ -0,0 +1,36 @@
+package httpserver
+
+import (
+	"encoding/json"
+	"net/http"
+	"testing"
+)
+
+func TestGetQuotedRemoteAddr(t *testing.T) {
+	f := func(remoteAddr, xForwardedFor, expectedAddr string) {
+		t.Helper()
+
+		req := &http.Request{
+			RemoteAddr: remoteAddr,
+		}
+		if xForwardedFor != "" {
+			req.Header = map[string][]string{
+				"X-Forwarded-For": {xForwardedFor},
+			}
+		}
+		addr := GetQuotedRemoteAddr(req)
+		if addr != expectedAddr {
+			t.Fatalf("unexpected remote addr;\ngot\n%s\nwant\n%s", addr, expectedAddr)
+		}
+
+		// Verify that the addr can be unmarshaled as JSON string
+		var s string
+		if err := json.Unmarshal([]byte(addr), &s); err != nil {
+			t.Fatalf("cannot unmarshal addr: %s", err)
+		}
+	}
+
+	f("1.2.3.4", "", `"1.2.3.4"`)
+	f("1.2.3.4", "foo.bar", `"1.2.3.4, X-Forwarded-For: foo.bar"`)
+	f("1.2\n\"3.4", "foo\nb\"ar", `"1.2\n\"3.4, X-Forwarded-For: foo\nb\"ar"`)
+}