code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public static <T> Completable fromObservable(final ObservableSource<T> observable) {
          ObjectHelper.requireNonNull(observable, "observable is null");
          return RxJavaPlugins.onAssembly(new CompletableFromObservable<T>(observable));
      }
docs: Returns a Completable instance that subscribes to the given Observable, ignores all values and
  emits only the terminal event.
  <p>
  <img width="640" height="414" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.fromObservable.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code fromObservable} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param <T> the type of the Observable
  @param observable the Observable instance to subscribe to, not null
  @return the new Completable instance
  @throws NullPointerException if flowable is null