the method agg of KeyValueGroupedDataset has 4 TypedColumn limits

 why has the method  agg of KeyValueGroupedDataset has 4 TypedColumn parameters limit?because performace?too much timing?in my case, it needs more than 4 agg metrics at the same time, so I only agg 4 metrics and join them by key, so the join waste time. so I wish the agg method had more than 4 TypedColumn parameters