Add support for OTEL_SDK_DISABLED environment variable

[hannahhaering]

Implements #4155

Changes

• Implemented OTEL_SDK_DISABLED according to the OpenTelemetry specification
• Applied support across TracerProvider, MeterProvider, and LoggerProvider

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.32%. Comparing base (0ea6c23) to head (d35c752).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6568      +/-   ##
==========================================
- Coverage   86.67%   86.32%   -0.36%     
==========================================
  Files         258      258              
  Lines       11895    11910      +15     
==========================================
- Hits        10310    10281      -29     
- Misses       1585     1629      +44     

Flag	Coverage Δ
unittests-Project-Experimental	86.06% <100.00%> (-0.53%)	⬇️
unittests-Project-Stable	86.11% <100.00%> (-0.45%)	⬇️

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

Files with missing lines	Coverage Δ
...elemetry/Logs/Builder/LoggerProviderBuilderBase.cs	97.36% <100.00%> (+0.39%)	⬆️
...emetry/Metrics/Builder/MeterProviderBuilderBase.cs	97.56% <100.00%> (+0.33%)	⬆️
...lemetry/Trace/Builder/TracerProviderBuilderBase.cs	98.24% <100.00%> (+0.16%)	⬆️

... and 5 files with indirect coverage changes

[martincostello]

A better way to do this, IMHO, would be to:

1. Not set the value in the constructor
2. Let the test set the value it's interested in
3. Set it back to the original value when being disposed

You could use a similar pattern to this to implement it in a cross-cutting way, then the usage would be something like:

using (new EnvironmentVariableScope("OTEL_SDK_DISABLED", value))
{
    var builder = new LoggerProviderBuilderBase();

    using var provider = builder.Build();

    Assert.IsType(expected, provider);
}

[hannahhaering]

Thanks for the suggestion, I've done that.

Is there any way I can retry the workflow without a commit? Sometimes the build fails even though I have no errors (like right now).

[martincostello]

Not without write access to the repo, no.