Register metrics for custom-error-pages

Request count and timer weren't visible on /metrics endpoint since they
haven't been registered in prometheus DefaultRegister.

images/custom-error-pages/main.go

@@ -27,6 +27,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
@@ -63,6 +64,11 @@ const (
 	ErrFilesPathVar = "ERROR_FILES_PATH"
 )
 
+func init() {
+	prometheus.MustRegister(requestCount)
+	prometheus.MustRegister(requestDuration)
+}
+
 func main() {
 	errFilesPath := "/www"
 	if os.Getenv(ErrFilesPathVar) != "" {