feat: Add Span.SetData()

[cleptric]

This is needed for OTel (#537).

[codecov]

Codecov Report

Base: 84.21% // Head: 84.24% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (ae107cf) compared to base (52e1547).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
+ Coverage   84.21%   84.24%   +0.02%     
==========================================
  Files          23       23              
  Lines        2820     2825       +5     
==========================================
+ Hits         2375     2380       +5     
+ Misses        365      364       -1     
- Partials       80       81       +1     

Impacted Files	Coverage Δ
tracing.go	84.81% <100.00%> (+2.38%)	⬆️
integrations.go	89.01% <0.00%> (-2.32%)	⬇️
transport.go	77.80% <0.00%> (-1.41%)	⬇️

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