src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.COMPUTATION)
    public final Observable<T> throttleFirst(long windowDuration, TimeUnit unit) {
        return throttleFirst(windowDuration, unit, Schedulers.computation());
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.COMPUTATION)
    public final Observable<T> throttleFirst(long windowDuration, TimeUnit unit) {
        return throttleFirst(windowDuration, unit, Schedulers.computation());
    }
Returns an Observable that emits only the first item emitted by the source ObservableSource during sequential
time windows of a specified duration.
<p>
This differs from {@link #throttleLast} in that this only tracks passage of time whereas
{@link #throttleLast} ticks at scheduled intervals.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/throttleFirst.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code throttleFirst} operates by default on the {@code computation} {@link Scheduler}.</dd>
</dl>

@param windowDuration
time to wait before emitting another item after emitting the last item
@param unit
the unit of time of {@code windowDuration}
@return an Observable that performs the throttle operation
@see <a href="http://reactivex.io/documentation/operators/sample.html">ReactiveX operators documentation: Sample</a>