src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final ParallelFlowable<T> doOnRequest(@NonNull LongConsumer onRequest) {
        ObjectHelper.requireNonNull(onRequest, "onRequest is null");
        return RxJavaPlugins.onAssembly(new ParallelPeek<T>(this,
                Functions.emptyConsumer(),
                Functions.emptyConsumer(),
                Functions.emptyConsumer(),
                Functions.EMPTY_ACTION,
                Functions.EMPTY_ACTION,
                Functions.emptyConsumer(),
                onRequest,
                Functions.EMPTY_ACTION
                ));
    }
@CheckReturnValue
    @NonNull
    public final ParallelFlowable<T> doOnRequest(@NonNull LongConsumer onRequest) {
        ObjectHelper.requireNonNull(onRequest, "onRequest is null");
        return RxJavaPlugins.onAssembly(new ParallelPeek<T>(this,
                Functions.emptyConsumer(),
                Functions.emptyConsumer(),
                Functions.emptyConsumer(),
                Functions.EMPTY_ACTION,
                Functions.EMPTY_ACTION,
                Functions.emptyConsumer(),
                onRequest,
                Functions.EMPTY_ACTION
                ));
    }
Call the specified consumer with the request amount if any rail receives a request.

@param onRequest the callback
@return the new ParallelFlowable instance