src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.ERROR)
    @SchedulerSupport(SchedulerSupport.COMPUTATION)
    public final Flowable<T> sample(long period, TimeUnit unit) {
        return sample(period, unit, Schedulers.computation());
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.ERROR)
    @SchedulerSupport(SchedulerSupport.COMPUTATION)
    public final Flowable<T> sample(long period, TimeUnit unit) {
        return sample(period, unit, Schedulers.computation());
    }
Returns a Flowable that emits the most recently emitted item (if any) emitted by the source Publisher
within periodic time intervals.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/sample.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>This operator does not support backpressure as it uses time to control data flow.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code sample} operates by default on the {@code computation} {@link Scheduler}.</dd>
</dl>

@param period
the sampling rate
@param unit
the {@link TimeUnit} in which {@code period} is defined
@return a Flowable that emits the results of sampling the items emitted by the source Publisher at
the specified time interval
@see <a href="http://reactivex.io/documentation/operators/sample.html">ReactiveX operators documentation: Sample</a>
@see <a href="https://github.com/ReactiveX/RxJava/wiki/Backpressure">RxJava wiki: Backpressure</a>
@see #throttleLast(long, TimeUnit)