Clarify that a compression-eligible span must not be buffered on an already ended parent

[trentm]

Otherwise, if a compression-eligible span ends after its parent has ended, then buffering it on the parent will result in it (or its later sibling) not being reported.

https://github.com/elastic/apm-agent-nodejs/pull/2623#pullrequestreview-932417034 shows and discusses a Node.js example where a compression-eligible span ends after its parent has ended. (This isn't a far-fetched scenario in Node.js.)

Checklist

• May the instrumentation collect sensitive information, such as secrets or PII (ex. in headers)?
  • [ ] Yes
  • [ ] Add a section to the spec how agents should apply sanitization (such as sanitize_field_names)
  • [ ] No
  • [ ] Why?
  • [x] n/a
• [x] Create PR as draft
• [x] Approval by at least one other agent
• [x] Mark as Ready for Review (automatically requests reviews from all agents and PM via CODEOWNERS)
  • Remove PM from reviewers if impact on product is negligible
  • Remove agents from reviewers if the change is not relevant for them
• [x] Approved by at least 2 agents + PM (if relevant)
• [x] Merge after 7 days passed without objections
• [ ] Create implementation issues through the meta issue template (this will automate issue creation for individual agents)

[apmmachine]

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