Use unknown-${service.agent.name}-service instead of my-service

[tobiasstadler]

What does this PR do?

Fixes #2306

Checklist

• This is something else
  • I have updated CHANGELOG.asciidoc

[github-actions]

👋 @tobiasstadler Thanks a lot for your contribution!

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[apmmachine]

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• Start Time: 2021-12-17T16:39:24.193+0000

• Duration: 70 min 45 sec

• Commit: 147c788

Test stats 🧪

Test	Results
Failed	0
Passed	2639
Skipped	21
Total	2660

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[tobiasstadler]

@eyalkoren Since the issue is assigned to you, would you please have a look?

[SylvainJuge]

LGTM, thanks for handling this @tobiasstadler !

[SylvainJuge]

/test

[eyalkoren]

run elasticsearch-ci/docs

[tobiasstadler]

Thank You!