public NodeTimeseries keepGenerationAfterWarmup(long generation, Optional<Node> node) {
        Optional<Instant> generationChange = generationChange(generation);
        return keep(snapshot -> isOnGenerationAfterWarmup(snapshot, node, generation, generationChange));
    }
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