feat: support metrics for otelmux

[martinyonatann]

Address issue #6638

[linux-foundation-easycla]

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• ✅ login: martinyonatann / name: Martin Yonatan Pasaribu (593ee04, df075bd, dab2c5d, 88fbdf6, 59f22e4, ebf1079, 9d0e324, 225115d)
• ✅ login: dmathieu / name: Damien Mathieu (c2fe973)

[dmathieu]

Thank you for this! ❤️

This PR does a lot more than just setup semconv.
In order to allow for better reviews, could you split it into multiple PRs?
From a first look, I'd do at least four:

• Setup the body wrapper and use it.
• Change the default span name behavior.
• Introduce the semconv package, and use it with tracing only.
• Introduce metrics support.

Striving for atomic PRs (each PR changes a single thing) makes for much speeder reviews.

[martinyonatann]

thankyou for the feedback @dmathieu ! ❤️
You're absolutely right, this PR ended up covering more ground than initially intended. i appreciate your suggestion to split it into atomic PRs for easier review. i'll get started on breaking it down into the following :

• 1. Setting up the body wrapper and using it Setup body wrapper in otelmux #6650
• 2. Changing the default span name behavior.
• 3. introducing the semconv package and using it with tracing only. Migrate to semconv for otelmux and implement the package in tracing #6652
• 4. Adding metrics support.

This approach should make the review process smother and more efficient. i'll prioritize reorganizing these changes and will submit the individual PRs soon. Let me know if there are any additional points you'd like me to address.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.9%. Comparing base (6676f17) to head (c2fe973).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6648     +/-   ##
=======================================
+ Coverage   73.8%   73.9%   +0.1%     
=======================================
  Files        195     195             
  Lines      16627   16671     +44     
=======================================
+ Hits       12285   12335     +50     
+ Misses      3982    3978      -4     
+ Partials     360     358      -2     

Files with missing lines	Coverage Δ
...mentation/github.com/gorilla/mux/otelmux/config.go	100.0% <100.0%> (ø)
...trumentation/github.com/gorilla/mux/otelmux/mux.go	95.5% <100.0%> (+1.5%)	⬆️

... and 1 file with indirect coverage changes