Fix splunksql DB goroutine leak

[MrAlias]

fix #1676
fix #1677

The Open function from the splunksql instrumentation opens a database to determine the underlying connector or driver. It never closes this database. Update the function that returns the initial database in a way where a user closes the returned database also closes the initial database.

This resolves the issue of the database goroutine being abandoned.

[codecov-commenter]

Codecov Report

Base: 81.86% // Head: 81.93% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (77c5077) compared to base (a4073e9).
Patch coverage: 89.28% of modified lines in pull request are covered.

❗ Current head 77c5077 differs from pull request most recent head 5056744. Consider uploading reports for the commit 5056744 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1682      +/-   ##
==========================================
+ Coverage   81.86%   81.93%   +0.06%     
==========================================
  Files          63       63              
  Lines        2625     2651      +26     
==========================================
+ Hits         2149     2172      +23     
- Misses        428      430       +2     
- Partials       48       49       +1     

Flag	Coverage Δ
Linux	81.55% <89.28%> (+0.06%)	⬆️
Windows	78.14% <89.28%> (+0.11%)	⬆️
macOS	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
instrumentation/database/sql/splunksql/sql.go	86.66% <86.95%> (-0.52%)	⬇️
...nstrumentation/database/sql/splunksql/connector.go	80.00% <100.00%> (+10.00%)	⬆️

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[pellared]

Looks great 👍

PS. I spent about ~2 hours reviewing this PR... And I was adding and later deleting comments a few times 😆 But everything is well structured, tested, and the comments are great!