public NodeTimeseries keep(Predicate<NodeMetricSnapshot> filter) {
        return new NodeTimeseries(hostname, snapshots.stream()
                                                     .filter(snapshot -> filter.test(snapshot))
                                                     .toList());
    }
Returns the instant this changed to the given generation, or empty if no *change* to this generation is present
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/autoscale/NodeTimeseries.java