void waitForAllEventsToBeHandled() throws Exception {
        CompletableFuture<Void> future = new CompletableFuture<>();
        eventQueue.append(() -> future.complete(null));
        future.get();
    }
Install a list of publishers. When a publisher is installed, we will publish a MetadataDelta
to it which contains the entire current image.

@param newPublishers     The publishers to install.

@return                  A future which yields null when the publishers have been added, or
                         an exception if the installation failed.
metadata/src/main/java/org/apache/kafka/image/loader/MetadataLoader.java