Decode value in OTEL_RESOURCE_ATTRIBUTES environment variable

[xiang17]

Fixes #3395.

OTEL_RESOURCE_ATTRIBUTES environment variable should follow format of W3C Baggage: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/resource/sdk.md#specifying-resource-information-via-an-environment-variable

W3C Baggage value definition: https://www.w3.org/TR/baggage/#value

Any code points outside of the baggage-octet range MUST be percent-encoded. The percent code point (U+0025) MUST be percent-encoded.

Note, value MAY contain any number of the equal sign (U+003D) code points. Parsers MUST NOT assume that the equal sign is only used to separate key and value.

Changes

Please provide a brief description of the changes here.

• Decode percent-encoded value in OTEL_RESOURCE_ATTRIBUTES.
• Treat only the first equal sign (=) as key-value separator. The rest are part of value.

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 86.88%. Comparing base (f630c55) to head (c35f63e).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6737   +/-   ##
=======================================
  Coverage   86.88%   86.88%           
=======================================
  Files         259      259           
  Lines       12066    12068    +2     
=======================================
+ Hits        10483    10485    +2     
  Misses       1583     1583           

Flag	Coverage Δ
unittests-Project-Experimental	86.72% <100.00%> (-0.07%)	⬇️
unittests-Project-Stable	86.64% <100.00%> (-0.15%)	⬇️

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

Files with missing lines	Coverage Δ
...OpenTelemetry/Resources/OtelEnvResourceDetector.cs	100.00% <100.00%> (ø)

[rajkumar-rangaraj]

how is this different from #6461

[xiang17]

how is this different from #6461

The biggest difference is that this is not in the hot path. This PR is simply a bug fix for OTEL_RESOURCE_ATTRIBUTES.

The functionality is covered in that PR if that goes through. However, that one is in the hot path (baggage propagation) and needs lots of scrutiny.

That one also validates key and value doesn't contain invalid characters.

[rajkumar-rangaraj]

LGTM

[github-actions]

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