public double getAverage() {
        return count != 0 ? (sum / count) : last;
    }
Returns the average reading of this value in the time interval, or--if no
value has been set within this period--the value of 'last' from the
most recent interval where this metric was set.
container-core/src/main/java/com/yahoo/container/jdisc/state/GaugeMetric.java