protected Function<CompletableFuture<O>, O> futureConverter() {
        return futureConverter;
    }
The {@link Function} that converts a {@link CompletableFuture} of response
into a response, e.g. {@link HttpResponse#of(CompletionStage)}
and {@link RpcResponse#from(CompletionStage)}.
core/src/main/java/com/linecorp/armeria/client/UserClient.java