Fix otelhttp span names to match the semantic conventions

[dmathieu]

Closes #726

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.5%. Comparing base (975d448) to head (411f4ac).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8871   +/-   ##
=====================================
  Coverage   83.4%   83.5%           
=====================================
  Files        193     193           
  Lines      15850   15898   +48     
=====================================
+ Hits       13232   13285   +53     
+ Misses      2146    2141    -5     
  Partials     472     472           

Files with missing lines	Coverage Δ
.../go-restful/otelrestful/internal/semconv/server.go	84.9% <100.0%> (+0.6%)	⬆️
...m/gin-gonic/gin/otelgin/internal/semconv/server.go	84.9% <100.0%> (+0.6%)	⬆️
...com/gorilla/mux/otelmux/internal/semconv/server.go	84.9% <100.0%> (+0.6%)	⬆️
.../labstack/echo/otelecho/internal/semconv/server.go	84.9% <100.0%> (+0.6%)	⬆️
...httptrace/otelhttptrace/internal/semconv/server.go	84.9% <100.0%> (+0.6%)	⬆️
instrumentation/net/http/otelhttp/handler.go	92.1% <100.0%> (ø)
...ation/net/http/otelhttp/internal/semconv/server.go	84.9% <100.0%> (+0.6%)	⬆️

... and 2 files with indirect coverage changes

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[dashpole]

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[MrAlias]

This is close, but I think the new default formatter still misses a couple of cases needed to actually match the HTTP span-name rules.

The main issue is that it uses r.Pattern verbatim, while the rest of otelhttp already treats that value as needing normalization before it becomes http.route. There is also still a gap for nonstandard methods, where the span name should fall back to HTTP instead of the raw method string.

I’d prefer to fix those in this PR so the new default naming is aligned with the existing route/method normalization logic.

[dmathieu]

I have moved the span name computation into the semconv internal package, so we can reuse the http.route computation.

Requested changes have been applied.