feat(span): remove SetAttribute method

[stanleynguyen]

Resolves #1107

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Stanley Nguyen (deacc0f)

[codecov]

Codecov Report

Merging #1216 into master will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1216   +/-   ##
======================================
  Coverage    76.5%   76.6%           
======================================
  Files         133     133           
  Lines        5795    5790    -5     
======================================
  Hits         4437    4437           
+ Misses       1109    1104    -5     
  Partials      249     249           

Impacted Files	Coverage Δ
api/trace/api.go	57.1% <ø> (ø)
api/trace/noop_span.go	33.3% <ø> (ø)
api/trace/tracetest/mock_span.go	12.5% <ø> (ø)
api/trace/tracetest/span.go	97.3% <ø> (+1.2%)	⬆️
bridge/opentracing/internal/mock.go	71.0% <ø> (+0.6%)	⬆️
sdk/trace/span.go	93.1% <ø> (+1.4%)	⬆️
... and 2 more