@Override
    public String toString() {
        return "TopicMetadata(" +
            "id=" + id +
            ", name=" + name +
            ", numPartitions=" + numPartitions +
            ", partitionRacks=" + partitionRacks +
            ')';
    }
@return Every partition mapped to the set of corresponding available rack Ids of its replicas.
        An empty map is returned if rack information is unavailable for all partitions.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/TopicMetadata.java