Export StartSpanFromContextWithTracer

[yurishkuro]

Replaces/fixes #152

[codecov-io]

Codecov Report

Merging #214 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master     #214   +/-   ##
=======================================
  Coverage   59.32%   59.32%           
=======================================
  Files          14       14           
  Lines         831      831           
=======================================
  Hits          493      493           
  Misses        298      298           
  Partials       40       40

Impacted Files	Coverage Δ
gocontext.go	85.71% <50%> (ø)	⬆️

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