public Optional<NodeMetricSnapshot> last() {
        if (snapshots.isEmpty()) return Optional.empty();
        return Optional.of(snapshots.get(snapshots.size() - 1));
    }
Returns the last (newest) snapshot in this, or empty if there are none.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/autoscale/NodeTimeseries.java