Remove SpanOptions.AllowRoot

[XSAM]

Resolves #96

This PR also updates the example to indicate the parent span is optional.

[codecov]

Codecov Report

Merging #101 (27432ab) into main (2f96852) will decrease coverage by 0.5%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #101     +/-   ##
=======================================
- Coverage   80.4%   79.9%   -0.6%     
=======================================
  Files         12      12             
  Lines        675     658     -17     
=======================================
- Hits         543     526     -17     
  Misses       107     107             
  Partials      25      25             

Impacted Files	Coverage Δ
config.go	90.0% <ø> (ø)
conn.go	77.7% <100.0%> (-1.2%)	⬇️
connector.go	100.0% <100.0%> (ø)
rows.go	55.8% <100.0%> (-0.7%)	⬇️
stmt.go	78.9% <100.0%> (-1.4%)	⬇️
tx.go	100.0% <100.0%> (ø)
... and 1 more

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