private static <U> void toCompletableFuture(Future<U> future, UnmodifiableFuture<U> cf) {
        if (future.isSuccess()) {
            cf.doComplete(future.getNow());
        } else {
            cf.doCompleteExceptionally(future.cause());
        }
    }
Returns an {@link UnmodifiableFuture} which will be completed when the specified
Netty's {@link Future} is completed.
core/src/main/java/com/linecorp/armeria/common/util/UnmodifiableFuture.java