public Set<BrokerServer> runningBrokers() {
        return brokersInState(BrokerState.RUNNING::equals);
    }
Get the brokers that have a {@link BrokerState#RUNNING} state.

@return the set of {@link BrokerServer} instances that are running;
        never null but possibly empty
connect/runtime/src/test/java/org/apache/kafka/connect/util/clusters/EmbeddedKafkaCluster.java