Messaging: Clarify span name when destination is temporary/anonymous

[joaopgrassi]

Fixes #1532

Changes

• Make it explicit that when the destination is temporary/anonymous, {destination} should be omitted on the span name
• Define what to use for messaging.destination.name when the destination is temporary/anonymous

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.