Add since for Span.Kind constants and make SpanBuilder.setSpanKind not abstract.

[bogdandrutu]

No description provided.

[codecov-io]

Codecov Report

Merging #1226 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #1226      +/-   ##
============================================
+ Coverage     81.99%   81.99%   +<.01%     
  Complexity     1236     1236              
============================================
  Files           193      193              
  Lines          6026     6027       +1     
  Branches        558      558              
============================================
+ Hits           4941     4942       +1     
- Misses          936      937       +1     
+ Partials        149      148       -1

Impacted Files	Coverage Δ	Complexity Δ
api/src/main/java/io/opencensus/trace/Span.java	82.5% <ø> (ø)	12 <0> (ø)	⬇️
...src/main/java/io/opencensus/trace/SpanBuilder.java	94.44% <0%> (-5.56%)	4 <0> (ø)
...census/implcore/trace/export/SpanExporterImpl.java	93.15% <0%> (+1.36%)	8% <0%> (ø)	⬇️

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