Add instrumentation for the pgx package

[MrAlias]

• README
• End-to-end tests
• Unit tests
• Code complete
• Documented

[codecov-commenter]

Codecov Report

Merging #91 (45c6e56) into main (bad256c) will increase coverage by 0.92%.
The diff coverage is 96.00%.

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   80.14%   81.07%   +0.92%     
==========================================
  Files          20       21       +1     
  Lines         806      856      +50     
==========================================
+ Hits          646      694      +48     
- Misses        129      130       +1     
- Partials       31       32       +1     

Flag	Coverage Δ
Linux	79.90% <96.00%> (+0.99%)	⬆️
Windows	80.37% <96.00%> (+0.96%)	⬆️
macOS	79.20% <96.00%> (+1.04%)	⬆️

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Impacted Files	Coverage Δ
...trumentation/github.com/jackc/pgx/splunkpgx/sql.go	96.00% <96.00%> (ø)

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