Switch the otelhttp semconv tests to use metricdatatest

[dmathieu]

This switches the otelhttp semconv tests to use the metricdatatest package rather than a custom struct with a partial metrics implementation.

This is meant to make it easier to start implementing the client and server metrics.

[dmathieu]

This PR may look big based on its diff size, but it's mostly just moving files into a test subpackage (to avoid adding the SDK as a dependency of otelhttp), and making some private methods public (since we're in an internal package, that has no public impact) so they can be used by the subpackage.

[codecov]

Codecov Report

Attention: Patch coverage is 88.63636% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.8%. Comparing base (637e1e5) to head (60a12df).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ion/net/http/otelhttp/internal/semconv/httpconv.go	80.0%	3 Missing ⚠️
...entation/net/http/otelhttp/internal/semconv/env.go	85.7%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6400   +/-   ##
=====================================
  Coverage   67.8%   67.8%           
=====================================
  Files        200     200           
  Lines      16589   16589           
=====================================
  Hits       11254   11254           
  Misses      4993    4993           
  Partials     342     342           

Files with missing lines	Coverage Δ
...ntation/net/http/otelhttp/internal/semconv/util.go	88.0% <100.0%> (ø)
...tion/net/http/otelhttp/internal/semconv/v1.20.0.go	93.8% <100.0%> (ø)
...entation/net/http/otelhttp/internal/semconv/env.go	90.4% <85.7%> (ø)
...ion/net/http/otelhttp/internal/semconv/httpconv.go	83.3% <80.0%> (ø)

[dmathieu]

Ping @open-telemetry/go-approvers for review.