@Override
    public String toString() {
        return "ClusterResource(clusterId=" + clusterId + ")";
    }
Return the cluster id. Note that it may be {@code null} if the metadata request was sent to a broker without
support for cluster ids. The first version of Kafka to support cluster id is 0.10.1.0.
clients/src/main/java/org/apache/kafka/common/ClusterResource.java