Is your feature request related to a problem? Please describe.

Context link apache/dubbo#15107

Describe the solution you'd like
Dubbo want use subscribe with callback anyway.

Describe alternatives you've considered
Normally, The usage should subscribe first and then get current all instances cached.
Or use get all instances with subscribe=false and then do subscribe.

But dubbo use get all instances with subscribe=true first to get and subscribe without subscriber, then call subscribe to add subscriber, which match the protection of duplicate subscribe for same services. So cause
apache/dubbo#15107.

For the old dubbo version users, dubbo community help nacos-client to do an enhancement for this usage situation which callback subscriber anyway when subscriber add.

Additional context
Add any other context or screenshots about the feature request here.