Change SetTag helpers in ext to public

[blakewedwards]

I left the SpanKindEnum one private since it seemed less reusable, but I can include it as well if preferred.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@17f6344). Click here to learn what that means.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##             master     #229   +/-   ##
=========================================
  Coverage          ?   60.07%           
=========================================
  Files             ?       15           
  Lines             ?      844           
  Branches          ?        0           
=========================================
  Hits              ?      507           
  Misses            ?      297           
  Partials          ?       40

Impacted Files	Coverage Δ
ext/tags.go	80% <66.66%> (ø)

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