Setup body wrapper in otelmux

[martinyonatann]

setup and implementation of a body wrapper in the go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux package

This PR addresses feedback provided in #6648

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 9 lines in your changes missing coverage. Please review.

Project coverage is 73.1%. Comparing base (f1bdc8b) to head (9b3f581).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...trumentation/github.com/gorilla/mux/otelmux/mux.go	76.6%	5 Missing and 2 partials ⚠️
...rilla/mux/otelmux/internal/request/body_wrapper.go	93.7%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6650     +/-   ##
=======================================
+ Coverage   73.0%   73.1%   +0.1%     
=======================================
  Files        189     191      +2     
  Lines      15773   15858     +85     
=======================================
+ Hits       11519   11598     +79     
- Misses      3920    3925      +5     
- Partials     334     335      +1     

Files with missing lines	Coverage Δ
...ux/otelmux/internal/request/resp_writer_wrapper.go	100.0% <100.0%> (ø)
...rilla/mux/otelmux/internal/request/body_wrapper.go	93.7% <93.7%> (ø)
...trumentation/github.com/gorilla/mux/otelmux/mux.go	85.0% <76.6%> (-4.3%)	⬇️

[akats7]

Does this still apply, since the way you wrap the default writer preserves the original implemented interfaces. Why would you want to implement them in RespWriterWrapper directly.

[dmathieu]

Note that this package is a copy from the otelhttp one.
https://github.com/open-telemetry/opentelemetry-go-contrib/tree/main/instrumentation/net/http/otelhttp/internal/request

Which makes me think maybe it should be templatized instead, to ensure things remain in sync.

[akats7]

lgtm