otelhttp: Introduce internal wrapper package

[dmathieu]

I am looking into cleaning up the otelhttp package, with the end goal of having separate handlers/transports for metrics and traces. (with #2782 in mind).

This is the first PR towards that goal. It moves the bodyWrapper and respWriterWrapper into a wrapper package, with additionnal tests, so the utility structs can be used across a future transport and handler package.

[codecov]

Codecov Report

Attention: Patch coverage is 90.56604% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 61.2%. Comparing base (6137d41) to head (6409b0f).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5291   +/-   ##
=====================================
  Coverage   61.2%   61.2%           
=====================================
  Files        185     186    +1     
  Lines      11207   11205    -2     
=====================================
  Hits        6865    6865           
+ Misses      4142    4140    -2     
  Partials     200     200           

Files	Coverage Δ
...ntation/net/http/otelhttp/internal/wrapper/body.go	100.0% <100.0%> (ø)
.../http/otelhttp/internal/wrapper/response_writer.go	100.0% <100.0%> (ø)
instrumentation/net/http/otelhttp/transport.go	97.4% <100.0%> (ø)
instrumentation/net/http/otelhttp/handler.go	87.0% <80.0%> (-0.2%)	⬇️

[dashpole]

I think we should consider waiting to do refactoring work until after we have updated to the stable semconv: #5092

[dmathieu]

Closing for now. I'll bring this back once the semconv migration is done.