Add db.operation.name attribute

[alanwest]

I originally removed the db.operation.name attribute in #2693, but forgot to add it back after the final result of this discussion #2693 (comment).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.97%. Comparing base (71655ce) to head (b016054).
Report is 862 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2800       +/-   ##
===========================================
+ Coverage   73.91%   87.97%   +14.05%     
===========================================
  Files         267        9      -258     
  Lines        9615      449     -9166     
===========================================
- Hits         7107      395     -6712     
+ Misses       2508       54     -2454     

Flag	Coverage Δ
unittests-Instrumentation.SqlClient	87.97% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...qlClient/Implementation/SqlActivitySourceHelper.cs	90.90% <100.00%> (ø)
...ient/Implementation/SqlClientDiagnosticListener.cs	86.61% <100.00%> (ø)

... and 268 files with indirect coverage changes

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