private Set<String> topics() {
            return updateResponse.topicMetadata().stream()
                    .map(MetadataResponse.TopicMetadata::topic)
                    .collect(Collectors.toSet());
        }
The RequestMatcher provides a way to match a particular request to a response prepared
through {@link #prepareResponse(RequestMatcher, AbstractResponse)}. Basically this allows testers
to inspect the request body for the type of the request or for specific fields that should be set,
and to fail the test if it doesn't match.
clients/src/test/java/org/apache/kafka/clients/MockClient.java