oteslog: Improve transforming slog.KindAny attributes

[pellared]

Reuse convertValue in otelslog.

Follows #6237.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.1%. Comparing base (3074e86) to head (f990508).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6254     +/-   ##
=======================================
+ Coverage   66.9%   67.1%   +0.2%     
=======================================
  Files        190     191      +1     
  Lines      12540   12621     +81     
=======================================
+ Hits        8395    8476     +81     
  Misses      3855    3855             
  Partials     290     290             

Files with missing lines	Coverage Δ
bridges/otellogrus/hook.go	97.5% <ø> (ø)
bridges/otelslog/convert.go	100.0% <100.0%> (ø)
bridges/otelslog/handler.go	96.4% <100.0%> (-0.1%)	⬇️
bridges/otelzap/core.go	97.2% <ø> (ø)