code: public final Mono<T> doAfterTerminate(Runnable afterTerminate) {
  		Objects.requireNonNull(afterTerminate, "afterTerminate");
  		return doAfterSuccessOrError((s, e)  -> afterTerminate.run());
  	}
docs: Add behavior (side-effect) triggered after the {@link Mono} terminates, either by
  completing downstream successfully or with an error.
  <p>
  <img class="marble" src="doc-files/marbles/doAfterTerminateForMono.svg" alt="">
  
  @param afterTerminate the callback to call after {@link Subscriber#onComplete} or {@link Subscriber#onError}
  
  @return an observed  {@link Flux}