Exporter spec fixup 2

[CodeBlanch]

Changes

• Zipkin error tag (added in SetStatus must store the StatusCode string value. #1571) value is now an empty string.
• Status enum values (discussed in Exporter spec updates. #1609) are now UPPERCASE to be in line with the exporter spec. Parsing made case insensitive so we'll be somewhat forgiving of people setting otel.status_code directly via Activity API.

Should match up to open-telemetry/opentelemetry-specification#1257 when merged.

TODOs

• CHANGELOG.md updated for non-trivial changes

[codecov]

Codecov Report

Merging #1620 (77334ae) into master (fe9e90d) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1620      +/-   ##
==========================================
+ Coverage   81.79%   81.83%   +0.04%     
==========================================
  Files         245      245              
  Lines        6575     6579       +4     
==========================================
+ Hits         5378     5384       +6     
+ Misses       1197     1195       -2     

Impacted Files	Coverage Δ
...Telemetry.Api/Internal/ActivityHelperExtensions.cs	100.00% <100.00%> (ø)
src/OpenTelemetry.Api/Internal/StatusHelper.cs	100.00% <100.00%> (ø)
src/OpenTelemetry.Api/Trace/ActivityExtensions.cs	94.11% <100.00%> (ø)
....Jaeger/Implementation/JaegerActivityExtensions.cs	95.62% <100.00%> (+0.06%)	⬆️
...metryProtocol/Implementation/ActivityExtensions.cs	86.84% <100.00%> (ø)
...plementation/ZipkinActivityConversionExtensions.cs	98.78% <100.00%> (+0.03%)	⬆️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+5.88%)	⬆️

[alanwest]

Looks Ok to me... urm, i mean OK