support distinct in xoptest, add tests

[muir]

No description provided.

[codecov-commenter]

Codecov Report

Merging #40 (c1400e8) into main (9cd6b2b) will decrease coverage by 0.32%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   44.50%   44.17%   -0.33%     
==========================================
  Files          37       37              
  Lines        3305     3436     +131     
==========================================
+ Hits         1471     1518      +47     
- Misses       1768     1854      +86     
+ Partials       66       64       -2     

Flag	Coverage Δ
go_tests	44.17% <0.00%> (-0.33%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
xoptest/testlogger.go	43.82% <0.00%> (-11.55%)	⬇️
xopjson/attributes.go	37.79% <0.00%> (+6.45%)	⬆️
span.go	44.06% <0.00%> (+20.33%)	⬆️

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