fromCompletionStage has been deprecated in 2.6.0, use completionStage instead.
Send the single value of the
CompletionStage when it completes and there is demand. If the
CompletionStage completes with
null stage is completed without emitting a value. If the
CompletionStage fails the stream is failed with that exception.
Reactive Streams semantics
emits the future completes
completes after the future has completed