Consolidate NullableAttributes across OpenTelemetry and OpenTelemetry.Api

[TimothyMothra]

Towards #4338

1. NullableAttributes are copied from dotnet runtime (link) to support older frameworks.
   This repo currently has nullable attributes spread across both OpenTelemetry project (link) and OpenTelemetry.Api (link).
   This PR merges these into a single file.

2. Another issue that needed to be addressed is target versions between OpenTelemetry and OpenTelemetry.Api projects.
   OpenTelemetry project targets net6.0;netstandard2.1;netstandard2.0;net462 and has a dependency on OpenTelemetry.Api which targets netstandard2.0;net462.
   Both projects need access to NullableAttributes and OpenTelemetry.Api makes its internals visible to OpenTelemetry.
   Consider that the highest version of OpenTelemetry.Api is netstandard2.0, which means the NullableAttributes are always present.
   The OpenTelemetry project will have access to these NullableAttributes in the Api project and will conflict with the dotnet runtime.
   To address this, OpenTelemetry.Api needs to target netstandard2.1 so it produces a dll without those internals. See also this comment: SDK: Nullable annotations for base classes & batch + shims to enable compiler features #3374 (comment)

Changes

Please provide a brief description of the changes here.

• add netstandard2.1 to OpenTelemetry.Api project.
• move NullableAttributes from OpenTelemetry to OpenTelemetry.Api.
• remove nullable attributes from OpenTelemetry.Api's Guard.cs and add to NullableAttributes.

Merge requirement checklist

• CONTRIBUTING guidelines followed (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

Merging #4293 (c6534a7) into main (6918866) will decrease coverage by 0.06%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4293      +/-   ##
==========================================
- Coverage   84.77%   84.72%   -0.06%     
==========================================
  Files         298      298              
  Lines       11984    11984              
==========================================
- Hits        10159    10153       -6     
- Misses       1825     1831       +6     

Impacted Files	Coverage Δ
src/OpenTelemetry.Api/Internal/Guard.cs	100.00% <ø> (ø)
...c/OpenTelemetry.Api/Internal/NullableAttributes.cs	0.00% <ø> (ø)

... and 6 files with indirect coverage changes

[Kielek]

Please check https://github.com/open-telemetry/opentelemetry-dotnet/blob/9395b9e944cdae19a9823c0bc3dacb257c9c4863/src/OpenTelemetry/Internal/Shims/NullableAttributes.cs

I think that content from this file should be merged with existing one.

[TimothyMothra]

These should absolutely be merged!

I didn't know this file was here! This is probably what was causing conflicts in my nullable branch. Thanks for pointing this out! :)

[TimothyMothra]

I've run into an issue merging these two files and I'm still exploring options before I make a recommendation.

Both OpenTelemetry and OpenTelemetry.Api projects need access to nullables.
OTel has a build dependency on OTel.Api.
OTel.Api makes internals visible to OTel.

The blocker is that OTel targets net6.0;netstandard2.1;netstandard2.0;net462 where as OTel.Api targets netstandard2.0;net462.
OTel (net6 and netstardard2.1) will take a dependency on OTel.Api (netstandard2.0).
And then OTel (net6 and netstardard2.1) has a conflict with runtime. ☹️☹️☹️

Error CS0433 The type 'AllowNullAttribute' exists in both 'OpenTelemetry.Api, Version=1.0.0.0, Culture=neutral, PublicKeyToken=7bd6737fe5b67e3c' and 'System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' OpenTelemetry (net6.0)

[TimothyMothra]

The fix is to add netstandard2.1 to the OTel.Api project. I'll bring this up in the SIG meeting tomorrow.

[TimothyMothra]

Discussed this in today's SIG and there are no blockers with this approach. :)

It would be preferred if we can move shared files to a separate project, but this only works if we can also remove the internals visible to between OTel and OTel.Api. I'm going to take a day to explore if this would work.

[github-actions]

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[github-actions]

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[TimothyMothra]

I'm closing this PR in favor of making a new Shared project.
See: #4348