Cherry-pick #25599 to 7.x: [Metricbeat] Add Couchbase's Sync Gateway module

[kaiyan-sheng]

Cherry-pick of PR #25599 to 7.x branch. Original message:

WIP

What does this PR do?

This PR adds a Metricset for Sync Gateway https://docs.couchbase.com/sync-gateway/2.7/introduction.html Edge Computing in Couchbase

Why is it important?

Couchbase users can rely on some features provided by the Sync Gateway to sync Couchbase Lite and Servers. This Metricset allows detailed monitorization of the Sync Gateways services.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

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[elasticmachine]

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• Start Time: 2021-07-30T14:46:34.997+0000

• Duration: 94 min 9 sec

• Commit: ff543ac

Test stats 🧪

Test	Results
Failed	0
Passed	9510
Skipped	2434
Total	11944

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Test stats 🧪

Test	Results
Failed	0
Passed	9510
Skipped	2434
Total	11944

[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 backport_25599_7.x upstream/backport_25599_7.x
git merge upstream/7.x
git push upstream backport_25599_7.x