add metrics for proxy

[NatSau02]

Description

(https://github.com/ContentSquare/chproxy/issues/406)

Pull request type

Please check the type of change your PR introduces:

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Checklist

• [ ] Linter passes correctly
• [x] Add tests which fail without the change (if possible)
• [ ] All tests passing
• [x] Extended the README / documentation, if necessary

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Further comments

[NatSau02]

https://github.com/ContentSquare/chproxy/actions/runs/8136869333/job/22248660475?pr=407#step:3:24 linter fell on files that I didn't change, very strange

[mga-chka]

I'm pinging you just in case you didn't see the comments @NatSau02

[NatSau02]

@mga-chka thanks for your comments, I've solved all the threads