feat(instrumentation): add support for '_latest_experimental' semantic conventions opt-in value

[pichlermarc]

Which problem is this PR solving?

Important

Part of a prototype for open-telemetry/semantic-conventions#2928

Implements a utility for a new semconv stability opt-in value LATEST_EXPERIMENTAL that contains both stable, and in-development telemetry according to the latest semconv.

See open-telemetry/semantic-conventions#2928
This is based on the current state of open-telemetry/semantic-conventions#3173

This should help with two things:

• lifting existing instrumentation closer to a stable state by allowing them to move forward with the in-development specification, reducing the effort needed when the semantic conventions for that area become stable
• allowing prototyping of in-development semantic conventions so that they can eventually be marked as stable, without emitting that telemetry by default.

Disclosure of AI use: I used Github Copilot and Claude Sonnet 4.5 to fill in some of the tests.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Added unit tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.43%. Comparing base (681c136) to head (1dca242).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Lines        9521     9521           
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  Hits         9086     9086           
  Misses        435      435           

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