[repo] Replace .NET6 target with .NET9

[rajkumar-rangaraj]

Fixes #5806

Changes

Please provide a brief description of the changes here.

1. Removed .NET 6.0 Target Framework as the EOL for .NET 6.0 is approaching (Nov 12, 2024)
2. Added .NET 9.0 RC1 Support
3. Added PackageValidationBaselineFrameworkToIgnore to ignore validation against .NET 6.0, this has to be added, as previous versions of this package had .NET 6.0 support.

Next steps (follow up PRs)

• Update System.Diagnostics.DiagnosticSource and Microsoft.Extensions.* packages. (Update System.Diagnostics.DiagnosticSource and Microsoft.Extensions.* packages to .NET 9.0  #5846)
• Handle CA2022 TODOs (Fix CA2022 - Avoid inexact read with 'System.IO.FileStream.Read(byte[], int, int) #5845)

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 86.38%. Comparing base (6250307) to head (d6940fd).
Report is 318 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5832      +/-   ##
==========================================
+ Coverage   83.38%   86.38%   +2.99%     
==========================================
  Files         297      257      -40     
  Lines       12531    11168    -1363     
==========================================
- Hits        10449     9647     -802     
+ Misses       2082     1521     -561     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.17% <100.00%> (?)
unittests-Project-Stable	86.19% <100.00%> (?)
unittests-Solution	86.27% <100.00%> (?)
unittests-UnstableCoreLibraries-Experimental	85.70% <ø> (?)

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

Files with missing lines	Coverage Δ
...nTelemetry/Internal/SelfDiagnosticsConfigParser.cs	84.09% <100.00%> (ø)

... and 222 files with indirect coverage changes

[reyang]

LGTM, I think https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/CONTRIBUTING.md#development-environment needs to be updated to cover .NET 9 as part of the environment setup. Can be a separate PR though.

[Kielek]

1 I think that this PR should include information about these changes in all affected CHANGELOG files.
2. It can be follow up, but you can potentially include it also here. Review alle conditional compilation. At least NET7_0_OR_GREATER and NET8_0_OR_GREATER. I think that all of them can be converted just to NET. There were decision to simplify condition where possible and remove exact versions.
3. Please update test\OpenTelemetry.Instrumentation.W3cTraceContext.Tests\W3CTraceContextTests.cs around line 83. Tests are no longer executed on .NET6 so the TODO comment and the code can be simplified.

[Kielek]

One more thing, should we really add there net9? .NET8 will be supported longer than .NET8.
Previously there were no packages targeted .NET7. Ref: #5795

[rajkumar-rangaraj]

@CodeBlanch What are your thoughts on it?

[cijothomas]

I don't see any particular advantage with a net9 target...

[CodeBlanch]

Reminds me of this: #4918

Question being, why did we add net8.0 to everything? 🤣 At the time I tried to block that because it exploded the amount of public API files we had and maintaining those is a pain. But I went and changed how the public API stuff works so we don't need public API files for a TFM unless there are customizations for that TFM.

Where we are now I don't have an issue with adding net9.0. We did it for net8.0. Which I guess is to say I'm fine with each year adding the latest version as a target when we upgrade. Don't have a strong reason for that other than when users look at NuGet and they see explicit targets I guess it is strong indication we were intentional 🤷

/cc @alanwest

[xiang17]

Was net8.0 more valid to add because it's a LTS release? net7.0 end of support is before net6.0, so it'll be removed before net6.0 anyways. net8.0, however, will outlive net6.0. Not adding net8.0 will unavoidably leave a gap somewhere in the timeline.

[alanwest]

I don't see any particular advantage with a net9 target...

Me either, and as @CodeBlanch points out this is something we have pushed against before, and honestly I never understood the reasons for adding a bunch of targets.

It stems back to this comment #4591 (comment). But again, I never understood the benefits to the OpenTelemetry .NET project.

All that said, I'm fine just adding net9.0 everywhere and continuing to make that the pattern going forward. If we want to circle back to this decision then I'd prefer it be in the context of a conversation about removing all unnecessary targets and going back to the state we had before.

[rajkumar-rangaraj]

I think that this PR should include information about these changes in all affected CHANGELOG files.

The best place to update this would be the RELEASENOTES.md file. This impacts all the projects in this repository. We haven't previously included the removal or addition of .NET in the changelog. If its fine, we could update this in a follow-up PR

2. It can be follow up, but you can potentially include it also here. Review alle conditional compilation. At least NET7_0_OR_GREATER and NET8_0_OR_GREATER. I think that all of them can be converted just to NET. There were decision to simplify condition where possible and remove exact versions.

Created an issue to track - #5848

3. Please update test\OpenTelemetry.Instrumentation.W3cTraceContext.Tests\W3CTraceContextTests.cs around line 83. Tests are no longer executed on .NET6 so the TODO comment and the code can be simplified.

This is on my list; I will leave a TODO for this PR. We also need to identify similar code in the repo. Since we have a comment on this test, it was easier for us to find it.