private void map(K key, Integer brokerId) {
        lookupMap.remove(key);
        fulfillmentMap.put(new FulfillmentScope(brokerId), key);
    }
Associate a key with a brokerId. This is called after a response in the Lookup
stage reveals the mapping (e.g. when the `FindCoordinator` tells us the group
coordinator for a specific consumer group).
clients/src/main/java/org/apache/kafka/clients/admin/internals/AdminApiDriver.java