Fix callback registration bug

[MrAlias]

Fix #4884

Currently, the registration loop uses the loop variable in the anonymous function registered as a callback. This means all of these functions will reference the last callback in the iterated slice.

This fixes that issue by declaring a copy of the loop variable within the iteration scope.

[dashpole]

This seems pretty serious. Should we consider a patch release?

[MrAlias]

This seems pretty serious. Should we consider a patch release?

I was thinking so.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e3eb3f7) 0.0% compared to head (98a5c63) 82.6%.

Additional details and impacted files

@@           Coverage Diff           @@
##           main   #4888      +/-   ##
=======================================
+ Coverage      0   82.6%   +82.6%     
=======================================
  Files         0     232     +232     
  Lines         0   18870   +18870     
=======================================
+ Hits          0   15605   +15605     
- Misses        0    2975    +2975     
- Partials      0     290     +290     

Files	Coverage Δ
sdk/metric/meter.go	90.5% <100.0%> (ø)

... and 231 files with indirect coverage changes