[8.0](backport #29094) Add beat field back to beat.stats

[mergify]

This is an automatic backport of pull request #29094 done by Mergify.
Cherry-pick of 24f4410 has failed:

On branch mergify/bp/8.0/pr-29094
Your branch is up to date with 'origin/8.0'.

You are currently cherry-picking commit 24f4410b6.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   metricbeat/docs/fields.asciidoc
	modified:   metricbeat/module/beat/stats/_meta/fields.yml
	renamed:    metricbeat/tests/system/test_beat.py -> metricbeat/module/beat/test_beat.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   metricbeat/module/beat/fields.go

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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[elasticmachine]

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• Start Time: 2021-12-14T14:49:43.438+0000

• Duration: 87 min 24 sec

• Commit: 746ebfd

Test stats 🧪

Test	Results
Failed	0
Passed	9153
Skipped	2422
Total	11575

💚 Flaky test report

Tests succeeded.

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @sayden? 🙏

[sayden]

/test

[mergify]

This pull request has not been merged yet. Could you please review and merge it @sayden? 🙏

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.0/pr-29094 upstream/mergify/bp/8.0/pr-29094
git merge upstream/8.0
git push upstream mergify/bp/8.0/pr-29094

[mergify]

This pull request has not been merged yet. Could you please review and merge it @sayden? 🙏