docs/spec: require duration >= 0

[axw]

Motivation/summary

Until now the APM Server would accept negative transaction and span durations, which does not make sense. We should reject negative durations, so we don't record junk data.

Recording negative durations would impact our ability to use HDRHistogram in percentile aggregations: elastic/kibana#64758.

Checklist

• I have signed the Contributor License Agreement.
• My code follows the style guidelines of this project (run make check-full for static code checks and linting)
• I have rebased my changes on top of the latest master branch
  - [ ] I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated CHANGELOG.asciidoc

How to test these changes

I hacked the Go agent to allow it to send negative durations, set the duration to -1, and observed a schema validation error. Normally the Go agent does not allow this; it uses negative values as a sentinel for setting duration based on now-start.

Related issues

Closes #3715

[ghost]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request docs/spec: require duration >= 0 #3721 event]

• Start Time: 2020-04-30T04:05:04.743+0000

• Duration: 51 min 37 sec (3037487)

• Commit: 73b3ef5

Test stats 🧪

Test	Results
Failed	0
Passed	2999
Skipped	139
Total	3138

Steps errors

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• Name: Test Sync

  • Description: ./script/jenkins/sync.sh

  • Result: FAILURE

  • Duration: 3 min 55 sec<

  • Start Time: 2020-04-30T04:16:16.772+0000