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.

This essentially copies the text from the GenAI semantic conventions, but adds additional notes on precedence, where messaging_latest_experimental ranks highest. I suppose this could technically also be used in the future to allow for enabling new experimental attributes through a common mechanism after the spec becomes stable.

There are a few open questions though:
Q1: given that the messaging semconv is not stable yet, nobody can implement a compliant messaging and messaging/dup yet: should we reduce the text to just include messaging_latest_experimental like we do in the GenAI semconv?
Q2: should I copy the changes to each file in docs/messaging?

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)