Warn instead of Info log instrument conflict

[MrAlias]

From the specification:

When a duplicate instrument registration occurs, and it is not corrected with a View, a warning SHOULD be emitted.

This logging line was added prior to the Warn function being added. Update the logging to use Warn instead.

[codecov]

Codecov Report

Merging #4202 (8ec9919) into main (135c64f) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4202   +/-   ##
=====================================
  Coverage   83.3%   83.4%           
=====================================
  Files        181     181           
  Lines      13940   13940           
=====================================
+ Hits       11625   11629    +4     
+ Misses      2094    2090    -4     
  Partials     221     221           

Impacted Files	Coverage Δ
sdk/metric/pipeline.go	89.6% <100.0%> (ø)

... and 1 file with indirect coverage changes