Gin exporter: support reading Client IP from custom headers (and make sure proxy is trusted)

[ItalyPaleAle]

With Gin, it's possible to configure the server to read the Client IP from custom headers; examples include X-Real-Ip or CF-Connecting-IP. This PR makes it possible to set as span attribute the same IP that Gin reads.

Additionally, it makes sure that headers such as "X-Forwarded-For" are used only if Gin is configured to trust the upstream server

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ItalyPaleAle / name: Alessandro (Ale) Segala (e978ded, 66ab057, d2bac00, 0273b8c, 27ffeda, 83866e7, 66533fe, 2046f0b, 5ea8dec, 9149000, 5cc0025, 69fc65d, 3a07c40, 4b7a39a)

[dmathieu]

PS: Also fixed unit tests, where there were assertions inside handlers, which are executed in separate goroutines

Could you split that into another PR, to make things easier to review?

[ItalyPaleAle]

PS: Also fixed unit tests, where there were assertions inside handlers, which are executed in separate goroutines

Could you split that into another PR, to make things easier to review?

Done

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.8%. Comparing base (a6c3927) to head (9149000).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6095     +/-   ##
=======================================
+ Coverage   65.7%   65.8%   +0.1%     
=======================================
  Files        204     204             
  Lines      13088   13121     +33     
=======================================
+ Hits        8608    8643     +35     
+ Misses      4216    4215      -1     
+ Partials     264     263      -1     

Files with missing lines	Coverage Δ
...stful/otelrestful/internal/semconvutil/httpconv.go	96.1% <100.0%> (+<0.1%)	⬆️
...hub.com/emicklei/go-restful/otelrestful/restful.go	100.0% <100.0%> (ø)
...ation/github.com/gin-gonic/gin/otelgin/gintrace.go	82.6% <100.0%> (+0.4%)	⬆️
...gonic/gin/otelgin/internal/semconvutil/httpconv.go	96.1% <100.0%> (+<0.1%)	⬆️
...rilla/mux/otelmux/internal/semconvutil/httpconv.go	96.1% <100.0%> (+<0.1%)	⬆️
...trumentation/github.com/gorilla/mux/otelmux/mux.go	89.4% <100.0%> (ø)
...entation/github.com/labstack/echo/otelecho/echo.go	100.0% <100.0%> (ø)
...ack/echo/otelecho/internal/semconvutil/httpconv.go	96.1% <100.0%> (+<0.1%)	⬆️
...on.v1/otelmacaron/internal/semconvutil/httpconv.go	96.1% <100.0%> (+<0.1%)	⬆️
...ntation/gopkg.in/macaron.v1/otelmacaron/macaron.go	92.3% <100.0%> (ø)
... and 4 more

... and 1 file with indirect coverage changes

[dmathieu]

This will need a changelog entry.