CloudEvents should re-use existing HTTP or messaging conventions

[joaopgrassi]

Fixes #654

Changes

We are approaching the stability of the messaging conventions, and as discussed we want to clarify the relation between CloudEvents convention with messaging. The conclusion we have is that applications/instrumentations working with CloudEvents should generally follow either the HTTP conventions or messaging. Those already offer mechanisms for context propagation and also are prepared to handle asynchronous scenarios such as when CloudEvents are sent via messaging systems.

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]
• schema-next.yaml updated with changes to existing conventions.

[pyohannes]

I don't want to make this too complicated, so as long as we avoid duplication with messaging semantic conventions (which the current approach already does), I'm happy to approve this.

Just one additional remark below.

[joaopgrassi]

@lmolkova @pyohannes I took your suggestions and also from the discussions in the CloudEvents PR and tried to simplify things a bit. Would be great if you can take another look. Thanks!