fix wrong representation of status value in zipkin exporter

[NewReStarter]

fix #3287
Set the name of span status code to upper case, fromUnset, Ok, Error to UNSET, OK, ERROR without making breaking changes. Previous version of code names can still be parsed.

[codecov]

Codecov Report

Merging #3340 (910601a) into main (94ae231) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3340   +/-   ##
=====================================
  Coverage   77.9%   77.9%           
=====================================
  Files        164     164           
  Lines      11359   11361    +2     
=====================================
+ Hits        8859    8861    +2     
  Misses      2301    2301           
  Partials     199     199           

Impacted Files	Coverage Δ
exporters/zipkin/model.go	94.7% <100.0%> (+<0.1%)	⬆️

[dmathieu]

I'm not sure this is the proper fix. Even though UnmarshalJSON remains the same here, the code actually changes, and the specification does require those codes to be capitalized, but not upper-case.
See https://github.com/open-telemetry/opentelemetry-specification/blob/fa36d43c4fe19207e653470d4efb13ff4361f711/specification/trace/api.md#set-status

I think the uppercasing should happen within the zipkin exporter, not for all codes.

[MrAlias]

I think the uppercasing should happen within the zipkin exporter, not for all codes.

Agreed

[NewReStarter]

I think the uppercasing should happen within the zipkin exporter, not for all codes.

Agreed

Thx, I didn't notice the side effect. New version of this fix has been submitted, please help do the further review.