[Do not merge] Prototype fix otelgin multipartform

[flc1125]

This is a prototype example from the #6609 (comment) section.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.8%. Comparing base (a2c5420) to head (fe2099c).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8161   +/-   ##
=====================================
  Coverage   81.8%   81.8%           
=====================================
  Files        194     194           
  Lines      13362   13382   +20     
=====================================
+ Hits       10931   10956   +25     
+ Misses      2033    2029    -4     
+ Partials     398     397    -1     

Files with missing lines	Coverage Δ
...umentation/github.com/gin-gonic/gin/otelgin/gin.go	93.0% <100.0%> (+0.1%)	⬆️

... and 4 files with indirect coverage changes

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