public static ClusterNodesTimeseries empty() {
        return new ClusterNodesTimeseries(NodeList.of(), List.of());
    }
Returns the "peak load" in this: Which is for each load dimension,
the average of the highest reading for that dimension on each node.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/autoscale/ClusterNodesTimeseries.java