[7.15] Add integration links metricbeat docs (#28110)

[dedemorton]

Backports #28110 to 7.15 branch.

(Required manual cherrypick to run make update before opening PR.)

[elasticmachine]

💔 Build Failed

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Build stats

• Start Time: 2021-10-06T01:00:52.179+0000

• Duration: 17 min 14 sec

• Commit: d88b9e7

Steps errors

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metricbeat-lint - make -C metricbeat check;make -C metricbeat update;make -C x-pack/metricbeat che

• Took 3 min 44 sec . View more details here
• Description: make -C metricbeat check;make -C metricbeat update;make -C x-pack/metricbeat check;make -C x-pack/metricbeat update;make check-no-changes;

List files to upload

• Took 0 min 0 sec . View more details here
• Description: ls -l src/github.com/elastic/beats/build/system-tests-*.tar.gz

x-pack/metricbeat-lint - make -C x-pack/metricbeat check;make -C x-pack/metricbeat update;make -C

• Took 3 min 57 sec . View more details here
• Description: make -C x-pack/metricbeat check;make -C x-pack/metricbeat update;make -C metricbeat check;make -C metricbeat update;make check-no-changes;

List files to upload

• Took 0 min 0 sec . View more details here
• Description: ls -l src/github.com/elastic/beats/build/system-tests-*.tar.gz

Error signal

• Took 0 min 0 sec . View more details here
• Description: Error 'hudson.AbortException: script returned exit code 2'

❕ Flaky test report

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[elasticmachine]

Pinging @elastic/obs-docs (Team:Docs)

[dedemorton]

@elasticmachine run elasticsearch-ci/docs

[dedemorton]

jenkins run the tests please

[dedemorton]

/test

[dedemorton]

Doc build passes. There are no errors related to this docs-only PR, so I am merging it.

[v1v]

There are some linted errors in this PR for the metricbeats:

metricbeat-lint - make -C metricbeat check;make -C metricbeat update;make -C x-pack/metricbeat che

https://beats-ci.elastic.co//blue/rest/organizations/jenkins/pipelines/Beats/pipelines/beats/pipelines/PR-28136/runs/4/steps/1572/log/?start=0

x-pack/metricbeat-lint - make -C x-pack/metricbeat check;make -C x-pack/metricbeat update;make -C

https://beats-ci.elastic.co//blue/rest/organizations/jenkins/pipelines/Beats/pipelines/beats/pipelines/PR-28136/runs/4/steps/1580/log/?start=0

[v1v]

I wonder if the comment #28136 (comment) is too verbose or hard to understand.

Any suggestions to make this easier from your point of view?