stats: register newly implemented metrics with gob

engine/caches.go

@@ -109,6 +109,11 @@ func init() {
 	gob.Register(new(StatSum))
 	gob.Register(new(StatAverage))
 	gob.Register(new(StatDistinct))
+	gob.Register(new(StatHighest))
+	gob.Register(new(StatLowest))
+	gob.Register(new(StatREPSC))
+	gob.Register(new(StatREPFC))
+
 	// others
 	gob.Register([]any{})
 	gob.Register([]byte{})