internal/shared/semconvutil: set http.route from Request.Pattern

[axw]

If Request.Pattern is set (e.g. by net/http.ServeMux), then use it for setting the http.route attribute on server request spans.

Related to #6193

(But does not close that, since according to https://opentelemetry.io/docs/specs/semconv/http/http-spans/#name we SHOULD use http.route in the span name. I'm leaving that for another day.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.1%. Comparing base (bdfe050) to head (49886b7).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6905   +/-   ##
=====================================
  Coverage   76.1%   76.1%           
=====================================
  Files        220     220           
  Lines      21448   21484   +36     
=====================================
+ Hits       16332   16368   +36     
  Misses      4557    4557           
  Partials     559     559           

Files with missing lines	Coverage Δ
...stful/otelrestful/internal/semconvutil/httpconv.go	95.7% <100.0%> (+<0.1%)	⬆️
...gonic/gin/otelgin/internal/semconvutil/httpconv.go	95.7% <100.0%> (+<0.1%)	⬆️
...rilla/mux/otelmux/internal/semconvutil/httpconv.go	95.7% <100.0%> (+<0.1%)	⬆️
...ack/echo/otelecho/internal/semconvutil/httpconv.go	95.7% <100.0%> (+<0.1%)	⬆️
...ace/otelhttptrace/internal/semconvutil/httpconv.go	95.7% <100.0%> (+<0.1%)	⬆️
...net/http/otelhttp/internal/semconvutil/httpconv.go	95.7% <100.0%> (+<0.1%)	⬆️

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