otelhttp: Remove custom wrapper after handling request

[VirrageS]

Closes #6908

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.6%. Comparing base (3ea6585) to head (a3c36c8).
Report is 21 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6914   +/-   ##
=====================================
  Coverage   75.5%   75.6%           
=====================================
  Files        207     207           
  Lines      19319   19362   +43     
=====================================
+ Hits       14601   14651   +50     
+ Misses      4283    4275    -8     
- Partials     435     436    +1     

Files with missing lines	Coverage Δ
...trumentation/github.com/gorilla/mux/otelmux/mux.go	95.7% <100.0%> (+0.1%)	⬆️
instrumentation/net/http/otelhttp/handler.go	93.0% <100.0%> (+0.1%)	⬆️

... and 6 files with indirect coverage changes

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[dmathieu]

Could you add a changelog entry?

[dmathieu]

Should we do this every time we use the body wrapper?
https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-go-contrib%20NewBodyWrapper&type=code

[VirrageS]

Should we do this every time we use the body wrapper? https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-go-contrib%20NewBodyWrapper&type=code

I wasn't sure in the beginning as I didn't know gorilla/mux that much. But from what I see it is simple muxer. So yes, I think we should update it too.

[dmathieu]

Could you do that in this PR?

[VirrageS]

Yeah, it is done already :)

[dmathieu]

This code is missing tests.

[VirrageS]

Added. Turns out that we don't really need this code in gorilla/mux as internally gorilla/mux does the following:

...
	if r.Match(req, &match) {
		handler = match.Handler
		req = requestWithVars(req, match.Vars)
		req = requestWithRoute(req, match.Route)
	}
...

func requestWithVars(r *http.Request, vars map[string]string) *http.Request {
	ctx := context.WithValue(r.Context(), varsKey, vars)
	return r.WithContext(ctx)
}

func requestWithRoute(r *http.Request, route *Route) *http.Request {
	ctx := context.WithValue(r.Context(), routeKey, route)
	return r.WithContext(ctx)
}

and r.WithContext(ctx) does a shallow copy of the http.Request so that the processed request is different than the original request and therefore r.Body is a different pointer (but still pointing to the same memory). Thus, original r.Body is left untouched.

But I would still leave this code just in case something changes in the future in gorilla/mux and for consistency with net/http code.