public MetadataImageBuilder addRacks() {
        for (int i = 0; i < 4; i++) {
            delta.replay(new RegisterBrokerRecord().setBrokerId(i).setRack("rack" + i));
        }
        return this;
    }
Add rack Ids for 4 broker Ids.
<p>
For testing purposes, each broker is mapped
to a rack Id with the same broker Id as a suffix.
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/MetadataImageBuilder.java