Drop unsampled transactions when connected to APM Server 8.0+

[tobiasstadler]

What does this PR do?

Fixes #2276

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation

[github-actions]

👋 @tobiasstadler Thanks a lot for your contribution!

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• Start Time: 2022-01-17T16:39:31.937+0000

• Duration: 44 min 15 sec

• Commit: 8e15859

Test stats 🧪

Test	Results
Failed	0
Passed	2442
Skipped	16
Total	2458

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Tests succeeded.

🤖 GitHub comments

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[tobiasstadler]

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

[felixbarny]

Let's simplify the test setup a bit and make it more consistent with other tests.
Otherwise, LGTM

Note: I have not tested the suggestions.

[felixbarny]

/test
run elasticsearch-ci/docs

[tobiasstadler]

run elasticsearch-ci/docs

[felixbarny]

run elasticsearch-ci/docs