CommandCenter providers stop method but we can not get instance and invoke it

[fangjian0423]

Issue Description

Spring Boot expose /shutdown Endpoint to shutdown Application. Sentinel-Starter includes sentinel-transport-simple-http module and this module using SPI to trigger SimpleHttpCommandCenter.

After invoke /shutdown endpoint, the process will not exit because the ServerSocket created in SimpleHttpCommandCenter is alive.

So I can invoke CommandCenter#stop to shutdown ThreadPool if I can get CommandCenter instance.

Type: feature request

Describe what feature you want

The single CommandCenter in CommandCenterInitFunc maybe should be hold in some where.

So we can get it and invoke CommandCenter#stop at any time.

[sczyh30]

Thanks for your suggestion. I think we can add a CommandCenterProvider to resolve the CommandCenter API and cache the instance so that users can get the instance from it.

[fangjian0423]

Cool. I agree with you.

It is very easy to get cached CommandCenter from CommandCenterProvider and we can invoke any methods with it.

[cdfive]

@sczyh30 @fangjian0423 Thanks for discuss the detail, I learned and have a try.