[6.1] Backport initial metrics enablement

[edwardneal]

Description

This backports 66a67bc from #3575 to 6.1.

It's possible to start an application with the M.D.S EventSource already enabled, even before the static constructor for SqlClientEventSource has run. In this situation, the constructor for Log will run OnEventCommand with an enablement command before it finishes executing. This enablement tries to call Metrics.EnableEventCounters, but this hasn't been initialized yet - it relies on the SqlClientEventSource instance.

To break this cycle, we record the desired startup state of the metrics in this case, and pass it to the SqlClientMetrics constructor.

Issues

Fixes #3711.

Testing

The timing makes this difficult to test. In the end, I built a test application locally, referenced 6.1.2 and ran

dotnet counters monitor --counters Microsoft.Data.SqlClient.EventSource -- SqlClientTimeQuery.exe

This allowed me to reproduce the problem: the counters appear in 6.0, don't appear in 6.1.2, appear again in 7.0-preview2 and now also appear in my local build.

Could someone run CI please? @empowerRob you should be able to verify with the nupkg build artifact.

[cheenamalhotra]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.32%. Comparing base (7a7b54e) to head (9bb2e60).
⚠️ Report is 17 commits behind head on release/6.1.

Files with missing lines	Patch %	Lines
...oft/Data/SqlClient/Diagnostics/SqlClientMetrics.cs	66.66%	1 Missing ⚠️
...c/Microsoft/Data/SqlClient/SqlClientEventSource.cs	80.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (7a7b54e) and HEAD (9bb2e60). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (7a7b54e)	HEAD (9bb2e60)
addons	1	0

Additional details and impacted files

@@               Coverage Diff                @@
##           release/6.1    #3718       +/-   ##
================================================
- Coverage        69.69%   59.32%   -10.37%     
================================================
  Files              281      273        -8     
  Lines            62413    61434      -979     
================================================
- Hits             43500    36448     -7052     
- Misses           18913    24986     +6073     

Flag	Coverage Δ
addons	?
netcore	63.84% <75.00%> (-8.92%)	⬇️
netfx	60.97% <100.00%> (-8.27%)	⬇️

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