public NodeTimeseries keepAfter(Instant oldestTime) {
        return new NodeTimeseries(hostname,
                                  snapshots.stream()
                                           .filter(snapshot -> snapshot.at().equals(oldestTime) || snapshot.at().isAfter(oldestTime))
                                           .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