src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.PASS_THROUGH)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> doOnRequest(LongConsumer onRequest) {
        return doOnLifecycle(Functions.emptyConsumer(), onRequest, Functions.EMPTY_ACTION);
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.PASS_THROUGH)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> doOnRequest(LongConsumer onRequest) {
        return doOnLifecycle(Functions.emptyConsumer(), onRequest, Functions.EMPTY_ACTION);
    }
Modifies the source {@code Publisher} so that it invokes the given action when it receives a
request for more items.
<p>
<b>Note:</b> This operator is for tracing the internal behavior of back-pressure request
patterns and generally intended for debugging use.
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator doesn't interfere with backpressure which is determined by the source {@code Publisher}'s
backpressure behavior.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code doOnRequest} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param onRequest
the action that gets called when a Subscriber requests items from this
{@code Publisher}
@return the source {@code Publisher} modified so as to call this Action when appropriate
@see <a href="http://reactivex.io/documentation/operators/do.html">ReactiveX operators
documentation: Do</a>
@since 2.0