otelgin: Using c.FullPath() to set http.route attribute

[NeoCN]

As spanName can be customized with SpanNameFormatter, so the spanName may not be the same with http.route, e.g. the spanName can be GET /users/:id, but the http.route is /users/:id.
This PR using c.FullPath() to set http.route attribute in otelgin to keep http.route not affected by SpanNameFormatter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.8%. Comparing base (c932948) to head (b7e90fd).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5734   +/-   ##
=====================================
  Coverage   63.8%   63.8%           
=====================================
  Files        194     194           
  Lines      12238   12236    -2     
=====================================
- Hits        7818    7817    -1     
  Misses      4197    4197           
+ Partials     223     222    -1     

Files	Coverage Δ
...ation/github.com/gin-gonic/gin/otelgin/gintrace.go	81.1% <100.0%> (+0.8%)	⬆️