[sqlclient] Make SqlClientInstrumentation a singleton

[alanwest]

In an upcoming PR we will be adding metric instrumentation. That instrumentation will need to leverage the same DiagnosticSourceListener that is used for traces. This PR makes SqlClientInstrumentation a singleton because multiple instances of the instrumentation would create duplicate activities.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.36%. Comparing base (71655ce) to head (3e11aac).
Report is 587 commits behind head on main.

Files with missing lines	Patch %	Lines
...ent/Implementation/SqlEventSourceListener.netfx.cs	77.77%	2 Missing ⚠️
...ient/Implementation/SqlClientDiagnosticListener.cs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2305       +/-   ##
===========================================
+ Coverage   73.91%   86.36%   +12.44%     
===========================================
  Files         267        8      -259     
  Lines        9615      330     -9285     
===========================================
- Hits         7107      285     -6822     
+ Misses       2508       45     -2463     

Flag	Coverage Δ
unittests-Instrumentation.SqlClient	86.36% <91.66%> (?)

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

Files with missing lines	Coverage Δ
...trumentation.SqlClient/SqlClientInstrumentation.cs	90.90% <100.00%> (ø)
...ation.SqlClient/TracerProviderBuilderExtensions.cs	100.00% <100.00%> (ø)
...ient/Implementation/SqlClientDiagnosticListener.cs	83.72% <91.66%> (ø)
...ent/Implementation/SqlEventSourceListener.netfx.cs	70.42% <77.77%> (ø)

... and 268 files with indirect coverage changes

[Kielek]

@alanwest, I do not think that metrics should relay on activities. Please consider sampling scenario. It might lead to dropping measurements.

As I remember we have tried this approach in AspNetCore, then changed to separate listeners.

[alanwest]

I do not think that metrics should relay on activities. Please consider sampling scenario. It might lead to dropping measurements.

As I remember we have tried this approach in AspNetCore, then changed to separate listeners.

Metrics will not rely on activities. A single listener will be needed though because when both tracing and metrics are enabled, the duration of the span and metrics must be the same. A stopwatch will be used when only metrics are enabled or the span is sampled.

Aspnet core was a bit different as the span is always created by the native instrumentation.

[CodeBlanch]

LGTM