private synchronized void verifyNotFenced() {
        if (this.producerFenced) {
            throw new ProducerFencedException("MockProducer is fenced.");
        }
    }
Create a new mock producer with invented metadata.

Equivalent to {@link #MockProducer(Cluster, boolean, Partitioner, Serializer, Serializer) new MockProducer(Cluster.empty(), false, null, null, null)}
clients/src/main/java/org/apache/kafka/clients/producer/MockProducer.java