[draft] feat: Add metrics to genqclient

[ra-grover]

Adds prom metrics for GenqClient :

• A HistrogramVec for latency partitioned by operation, code and method.
• CounterVec for number of requests partitioned by operation, code and method
• CounterVec for errors (resp.Errors)

I have:

• [x] Written a clear PR title and description (above)
• [ ] Signed the Khan Academy CLA
• [ ] Added tests covering my changes, if applicable
• [ ] Included a link to the issue fixed, if applicable
• [ ] Included documentation, for new features
• [ ] Added an entry to the changelog

[ra-grover]

@benjaminjkraft Sorry that I haven't ✅ all the steps in the PR description, I just wanted to gauge the interest from the maintainers if they are interested in something like this. I am willing to complete the description/test cases once I have a nod from maintainers.

[benjaminjkraft]

Closing this since for housekeeping purposes -- happy to continue to discuss here or in an issue if it's helpful!