feat: prometheus metrics monitoring supported

[kiraxie]

Hi,

Does v2ray have any plan to support prometheus metrics monitoring in next version or further version?
Regarding the code base right now, it's already supported grpc, and looks like V5.x supported Restful query.
However, I found that it needs another layer to convert format for prometheus(with Grafana).
Actually, it's already had a repo which export the v2ray statistic to prometheus. But what if v2ray support prometheus directly? It would be perfect to see that!

So, I tried to add patch into current version, it support by v5 and v4 of configuration version at the same time. No any configuration format changed.

What do you think about the feature?

[kiraxie]

Hi, dose anyone can help to review my PR? Or any further suggestion?

In my view, it's more convivence to monitor the status of v2ray by Grafana if this PR approved.

[github-actions]

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