sentinel-apache-dubbo-adapter 在异步模式下永远取不到异常

代码：

https://github.com/alibaba/Sentinel/blob/4498de480f368d6bf63afbfda42797172c89573b/sentinel-adapter/sentinel-dubbo-adapter/src/main/java/com/alibaba/csp/sentinel/adapter/dubbo/SentinelDubboProviderFilter.java#L73

在 dubbo 2.7.5 后，所有`Filter`均支持异步操作，返回的 `Result` 对象通常是 `AsyncRpcResult`，对 `AsyncRpcResult` 调用`hasException()` 时如果异步任务未完成，则使用默认的返回值，永远不包含异常。

https://github.com/apache/dubbo/blob/9c49efeacfd87d2d4409fb000cebd58e1114ec8a/dubbo-rpc/dubbo-rpc-api/src/main/java/org/apache/dubbo/rpc/AsyncRpcResult.java#L143-L155

Start at sentinel-adapter/sentinel-dubbo-adapter/src/main/java/com/alibaba/csp/sentinel/adapter/dubbo/SentinelDubboProviderFilter.java#L73 and compare its result handling with the linked Apache Dubbo AsyncRpcResult implementation. Trace how asynchronous completion affects exception detection; the work is done when exceptions from asynchronous results are reliably observed instead of being missed.

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