[8.1] docs: Tail-based sampling (backport #7317)

[mergify]

This is an automatic backport of pull request #7317 done by Mergify.

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[apmmachine]

💔 Build Failed

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Build stats

• Start Time: 2022-03-09T01:01:08.510+0000

• Duration: 48 min 25 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4368
Skipped	14
Total	4382

Steps errors

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Restore files previously stashed

• Took 7 min 48 sec . View more details here
• Description: source

Checks if running on a Unix-like node

• Took 0 min 0 sec . View more details here

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