public ActiveInterval startNewInterval() {
        synchronized (monitor) {
            pushEndOfPeriodToNow();
            ActiveIntervalInfo activeIntervalInfo = new ActiveIntervalInfo(endOfPeriodNanos);
            activeIntervals.add(activeIntervalInfo);
            maxLoad = max(maxLoad, activeIntervals.size()) ;
            ++numIntervalsStarted;
            return () -> endInterval(activeIntervalInfo);
        }
    }
Starts a new (active) interval.  The caller MUST call {@link ActiveInterval#close()} on the
returned instance exactly once, which will end the interval.
zkfacade/src/main/java/com/yahoo/vespa/curator/stats/LatencyStats.java