code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.FULL)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <U> Flowable<T> delaySubscription(Publisher<U> subscriptionIndicator) {
          ObjectHelper.requireNonNull(subscriptionIndicator, "subscriptionIndicator is null");
          return RxJavaPlugins.onAssembly(new FlowableDelaySubscriptionOther<T, U>(this, subscriptionIndicator));
      }
docs: Returns a Flowable that delays the subscription to this Publisher
  until the other Publisher emits an element or completes normally.
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The operator forwards the backpressure requests to this Publisher once
  the subscription happens and requests Long.MAX_VALUE from the other Publisher</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>This method does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param <U> the value type of the other Publisher, irrelevant
  @param subscriptionIndicator the other Publisher that should trigger the subscription
  to this Publisher.
  @return a Flowable that delays the subscription to this Publisher
  until the other Publisher emits an element or completes normally.
  @since 2.0