public static <U> KafkaFuture<U> completedFuture(U value) {
        KafkaFuture<U> future = new KafkaFutureImpl<>();
        future.complete(value);
        return future;
    }
Returns a new KafkaFuture that is already completed with the given value.
clients/src/main/java/org/apache/kafka/common/KafkaFuture.java