Returns a Mono that terminates when the instance is terminated by any reason. Note, in
case of error termination, the cause of error will be propagated as an error signal through
Subscriber.onError(Throwable). Otherwise, Subscriber.onComplete() will be called.
a Mono to track completion with success or error of the underlying resource.
When the underlying resource is an `RSocket`, the Mono exposes stream 0 (i.e.
connection level) errors.