[8.10] Fix tail-based sampling discarding low throughput and low sample rate traces (backport #11642)

[mergify]

This is an automatic backport of pull request #11642 done by Mergify.
Cherry-pick of b0e8925 has failed:

On branch mergify/bp/8.10/pr-11642
Your branch is up to date with 'origin/8.10'.

You are currently cherry-picking commit b0e892589.
  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	modified:   x-pack/apm-server/sampling/groups.go
	modified:   x-pack/apm-server/sampling/groups_test.go
	modified:   x-pack/apm-server/sampling/processor_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   changelogs/head.asciidoc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[carsonip]

@kruskall oh no i meant to remove the changelog