Add support for asynchronous Servlet in Sentinel Web Servlet Filter

[xiejiashuai]

Issue Description

Type: feature request

Describe what happened (or what feature you want)

Support asynchronous Servlet in Sentinel Web Servlet Filter.

Spring Boot中使用 CompletableFuture<T> or DefferdResult<T> or Callable<T> 作为返回值，引入 Sentinel CommonFilter 后无法响应，且无法正确统计。是否支持两种形式（同步与异步 Servlet）并存，比如使用案例如下：

@RestController
public class AsyncController {

    @GetMapping("/async")
    public CompletableFuture<String> async() {

        return CompletableFuture.supplyAsync(() -> {

                    try {
                        TimeUnit.SECONDS.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    return "async\";
                }
        );

    }

}

[jasonjoo2010]

I am trying working on it.

So there're two plan to support it: Same filter or separate filters

Which one we prefer? @sczyh30 @xiejiashuai

In one filter to implement them all reflecting should be used.

[sczyh30]

I am trying working on it.

So there're two plan to support it: Same filter or separate filters

Which one we prefer? @sczyh30 @xiejiashuai

In one filter to implement them all reflecting should be used.

Maybe it's better to do this in the same filter, but seems a little tricky?

[xiejiashuai]

I am trying working on it. So there're two plan to support it: Same filter or separate filters Which one we prefer? @sczyh30 @xiejiashuai In one filter to implement them all reflecting should be used.

Maybe it's better to do this in the same filter, but seems a little tricky?

In my opinion ,One Filter seems impossible. Even more tricky，One application both has synchronized servlet and asychronous servlet.

[xiejiashuai]

I am trying working on it. So there're two plan to support it: Same filter or separate filters Which one we prefer? @sczyh30 @xiejiashuai In one filter to implement them all reflecting should be used.

Maybe it's better to do this in the same filter, but seems a little tricky?

Can only support that in Spring Mvc By AsyncHandlerInterceptor ?

[jasonjoo2010]

I am trying working on it. So there're two plan to support it: Same filter or separate filters Which one we prefer? @sczyh30 @xiejiashuai In one filter to implement them all reflecting should be used.

Maybe it's better to do this in the same filter, but seems a little tricky?

In my opinion ,One Filter seems impossible. Even more tricky，One application both has synchronized servlet and asychronous servlet.

One filter is indeed possible i think.

I make an implementation and testing now

[jasonjoo2010]

I post a PR #310

And still working on more unit tests

[xiejiashuai]

I post a PR #310

And still working on more unit tests

Very Thanks, I will see it.