Add monitor/main cluster privilege to package

[axw]

Motivation/summary

Add a cluster privilege to the apm package so apm-server can query the / Elasticsearch endpoint, as used in Stack Monitoring to fetch the cluster UUID.

Checklist

• Update CHANGELOG.asciidoc
  - [ ] Documentation has been updated

How to test these changes

TBD

Related issues

None

[apmmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-10-18T07:30:30.328+0000

• Duration: 40 min 50 sec

• Commit: 04afea1

Test stats 🧪

Test	Results
Failed	0
Passed	6289
Skipped	18
Total	6307

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[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.