Does provider "async = true" need to be remove from url when register to registry?

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.7.0-SNAPSHOT

Step to reproduce this issue

1. https://github.com/apache/incubator-dubbo/blob/master/dubbo-registry/dubbo-registry-api/src/main/java/org/apache/dubbo/registry/integration/RegistryProtocol.java

 private URL getRegisteredProviderUrl(final Invoker<?> originInvoker) {
        URL providerUrl = getProviderUrl(originInvoker);
        //The address you see at the registry
        return providerUrl.removeParameters(getFilteredKeys(providerUrl))
                .removeParameter(Constants.MONITOR_KEY)
                .removeParameter(Constants.BIND_IP_KEY)
                .removeParameter(Constants.BIND_PORT_KEY)
                .removeParameter(QOS_ENABLE)
                .removeParameter(QOS_PORT)
                .removeParameter(ACCEPT_FOREIGN_IP)
                .removeParameter(VALIDATION_KEY)
                .removeParameter(INTERFACES);
    }
TO
 private URL getRegisteredProviderUrl(final Invoker<?> originInvoker) {
        URL providerUrl = getProviderUrl(originInvoker);
       // 此处移除async标记，仅保留标记在provider端
        Set<String> keySet = providerUrl.getParameters().keySet();
        List<String> asyncKey = new ArrayList<>();
        for(String key : keySet) {
            if(key.endsWith("." + Constants.ASYNC_KEY) || key.equals(Constants.ASYNC_KEY)) {
                asyncKey.add(key);
            }
        }
        //The address you see at the registry
        return providerUrl.removeParameters(getFilteredKeys(providerUrl))
                .removeParameter(Constants.MONITOR_KEY)
                .removeParameter(Constants.BIND_IP_KEY)
                .removeParameter(Constants.BIND_PORT_KEY)
                .removeParameter(QOS_ENABLE)
                .removeParameter(QOS_PORT)
                .removeParameter(ACCEPT_FOREIGN_IP)
                .removeParameter(VALIDATION_KEY)
                .removeParameter(INTERFACES)
                .removeParameters(asyncKey);
    }

that if Provider use “provider async” like this：

 public String sayHello(String name) {
        System.out.println("Main sayHello() method start.");
        final AsyncContext asyncContext = RpcContext.startAsync();
        new Thread(() -> {
            asyncContext.signalContextSwitch();
            System.out.println("Attachment from consumer: " + RpcContext.getContext().getAttachment("consumer-key1"));
            System.out.println("    -- Async start.");
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            asyncContext.write("Hello " + name + ", response from provider.");
            System.out.println("    -- Async end.");
        }).start();
        System.out.println("Main sayHello() method end.");
        return "hello, " + name;
    }
 <dubbo:method name="sayHello" async="true"/>

and consumer user dubbo version low than 2.7.0-SNAPSHOT, “async = true” will know from provider registed url, that need consumer use old async to invoke this method. But it is not what we expected, we think async should only be set by consumer itself. 英文不太好，中文版：

1. Provider版本使用 2.7.0-SNAPSHOT版本，然后发布接口使用了"provider async"的方式（见上面第二段代码）,则需要在发布的时候添加 <dubbo:method name="sayHello" async="true"/>则会将 async=true 的标记注册到注册中心，这时候就会将标记透传到 Consumer 端，导致服务调用变成异步的，而出现返回值为 null 的情况。
2. 根据我个人的理解，async 标记应该仅由 provider 或 consumer 自己指定，而不应该透传到对方感知。

[fingthinking]

@carryxyh 如果在 Consumer 端移除该属性，则有一点要求是 provider 必须在 consumer 后面升级，否则会将标记透传过去，造成consumer 端变成异步调用。

[chickenlj]

Therefore, for compatibility reasons, it is necessary to fix the problem on the Provider side so that method.async is not registered to the registry.

We are working on this in branch dev-meta.

In the Consumer, ignoring the Provider’s method.async is equivalent to an insurance, ensuring that the Provider’s method.async will not affect the Consumer’s behavior under any circumstances.

I have seen that you fixed this part in #2434. I tend to merge it now.