It is different to monitor how/if your DNSs are running than if the world is resolving your domain/server names, they return v4/v6 records or even if what they

It is different to monitor how/if your DNSs are running than if the world is resolving your domain/server names, they return v4/v6 records or even if what they are resolving is in the right IP blocks. It is up to what you really need to know. "the world" is a bit too wide, using some particular ones like in the example may trigger false alarms if they can't be reached or they have some internal problem. But for widely enough used ones that may be something to be aware of anyway.