messaging: add messaging_latest_experimental opt-in value

[pichlermarc]

Fixes #2928

Changes

Adds messaging_latest_experimental as a possible value to SEMCONV_STABILITY_OPT_IN for instrumentations that want to update to the latest semconv without breaking end-users while messaging semconv is still in-development.

The updated spec is also written in a way that once the messaging spec becomes stable, there's an opt-in flag for enabling the latest in-development semconv (attributes for instance) without having it on by default.

Note: I think the name of the flag could be aligned to be messaging/latest-experimental to follow a similar format to messaging/dup, but choose to align with the GenAI semconv for now where genai_latest_experimental is already used.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• Links to the prototypes or existing instrumentations (when adding or changing conventions)

[joaopgrassi]

Should we be crystal clear here and say that this only emits stable conventions? Because it says "stop emitting the old, experimental. I kinda don't like "old" here because there might be "new" unstable conventions as well, and are those included or not? :D

Maybe we just say: emit only stable messaging conventions. and keep it short.

Could also be done as a follow up. I just wanted to bring here now for discussions. CC @open-telemetry/specs-semconv-maintainers

[pichlermarc]

I kinda don't like "old" here because there might be "new" unstable conventions as well, and are those included or not? :D

Heh, good point! I'm happy to follow-up on it if we decide to go that route. 🙂

Maybe we just say: emit only stable messaging conventions. and keep it short.

I like that. Maybe that small change could be the resolution for #3200 🙂

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