Warn on intitialization of Simple SpanProcessor

[dubonzi]

Ended up not creating a Warn function as mentioned in the issue since Info is already being used for warning messages in other places.

Resolves #1875

[dmathieu]

This should have a changelog entry.

[codecov]

Codecov Report

Merging #3854 (1503ce5) into main (de497de) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3854     +/-   ##
=======================================
- Coverage   81.8%   81.7%   -0.1%     
=======================================
  Files        170     170             
  Lines      12909   12911      +2     
=======================================
- Hits       10561   10559      -2     
- Misses      2129    2133      +4     
  Partials     219     219             

Impacted Files	Coverage Δ
sdk/trace/simple_span_processor.go	75.4% <100.0%> (+0.8%)	⬆️

... and 2 files with indirect coverage changes

[pellared]

Ended up not creating a Warn function as mentioned in the issue since Info is already being used for warning messages in other places.

I will create an issue for it.

EDIT: #3866

[dubonzi]

I just rebased and switched to using the new Warn function added in #3900.