private Optional<Instant> generationChange(long targetGeneration) {
        if (snapshots.isEmpty()) return Optional.empty();
        if (snapshots.get(0).generation() == targetGeneration) return Optional.of(snapshots.get(0).at());
        for (NodeMetricSnapshot snapshot : snapshots) {
            if (snapshot.generation() == targetGeneration)
                return Optional.of(snapshot.at());
        }
        return Optional.empty();
    }
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