otelgin: refine span name formatting and deprecate SpanNameFormatter option

[flc1125]

source: #6375

supplements:

• The Gin framework does not support mixed case or lowercase methods. Therefore, I removed this part of the single test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.0%. Comparing base (0fc2c4c) to head (1d9b5f5).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6381   +/-   ##
=====================================
  Coverage   67.0%   67.0%           
=====================================
  Files        193     193           
  Lines      15660   15674   +14     
=====================================
+ Hits       10494   10514   +20     
+ Misses      4876    4872    -4     
+ Partials     290     288    -2     

Files with missing lines	Coverage Δ
...ation/github.com/gin-gonic/gin/otelgin/gintrace.go	89.6% <100.0%> (-0.3%)	⬇️
...ntation/github.com/gin-gonic/gin/otelgin/option.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[dmathieu]

Based on the ongoing discussions with the need or not for SpanNameFormatter, could you update this PR to only include the default span name?

[flc1125]

Based on the ongoing discussions with the need or not for SpanNameFormatter, could you update this PR to only include the default span name?

OK, I will add another draft version of the PR (which may not have passed unit testing and CI).

[flc1125]

@dmathieu A draft version: #6396