public Sample oldest(long now) {
        if (samples.isEmpty())
            this.samples.add(newSample(now));
        Sample oldest = this.samples.get(0);
        for (int i = 1; i < this.samples.size(); i++) {
            Sample curr = this.samples.get(i);
            if (curr.startTimeMs < oldest.startTimeMs)
                oldest = curr;
        }
        return oldest;
    }
A SampledStat records a single scalar value measured over one or more samples. Each sample is recorded over a
configurable window. The window can be defined by number of events or elapsed time (or both, if both are given the
window is complete when <i>either</i> the event count or elapsed time criterion is met).
<p>
All the samples are combined to produce the measurement. When a window is complete the oldest sample is cleared and
recycled to begin recording the next sample.

Subclasses of this class define different statistics measured using this basic pattern.
clients/src/main/java/org/apache/kafka/common/metrics/stats/SampledStat.java