dubbo MetricsFilter 获取不到线程池统计数据

[welcc]

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

Environment

• Dubbo version: 2.7.9
• Operating System version: mac os
• Java version: 1.8.0_281

Steps to reproduce this issue

1. 部署新版dubo-admin
2. 在dubbo provider开启metric filter dubbo:
   provider: filter: metrics metrics: protocol: dubbo port: 20880
3. 在dubbo-admin服务统计能够获取服务调用统计数据，但是获取不到线程池统计数据

问题原因排查： 1.查看dubbo源码(org.apache.dubbo.monitor.dubbo.MetricsFilter)发现获取线程池统计数据实现过程是这样的，从DataStore中获取线程池，然后获取线程池数据： private List getThreadPoolMessage() { DataStore dataStore = ExtensionLoader.getExtensionLoader(DataStore.class).getDefaultExtension(); Map<String, Object> executors = dataStore.get(EXECUTOR_SERVICE_COMPONENT_KEY); ... } 2.但是在 https://github.com/apache/dubbo/commit/2484877189c89e12c0ff67504838b702153aa016#diff-45baf3c442a606b31ed84241935cf0cbc571a532cad96bf9b27b4365fb1f0db6 （Reduce context switching cost by optimizing thread model on consumer …）这次commit中去掉了将创建的线程池放入DataStore中的代码：

3.改为用org.apache.dubbo.common.threadpool.manager.DefaultExecutorRepository存储线程池：

这样导致MetricsFilter中获取线程池监控数据的逻辑失效。

4.并且DefaultExecutorRepository中存储线程池的容器为private，没有提供外部获取方式，只能通过getExecutor(URL url) 获取 但是这个根据url获取线程池的方式在MetricsFilter无法使用

[laddcn]

I have fixed this bug yet. Please help me for review.

[ilaotan]

这货都开始标识过期了,也没写替代的类,哎

[chickenlj]

Observability under construction in 3.1.x version, progresses will be updated there.