Cherry-pick #23887 to 7.x: Check fields are documented for aws metricsets

[kaiyan-sheng]

Cherry-pick of PR #23887 to 7.x branch. Original message:

What does this PR do?

This PR is to unskip flaky test in s3_request metricset and add check on fields to see if they are documented: #21826

This PR also add TestMetricsetFieldsDocumented for fields from rds, s3_daily_storage, s3_request, sqs and cloudwatch metricsets: #17337

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.

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Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #24068 opened

• Start Time: 2021-02-16T17:54:04.577+0000

• Duration: 36 min 50 sec

• Commit: b301436

Test stats 🧪

Test	Results
Failed	0
Passed	2455
Skipped	216
Total	2671

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💚 Flaky test report

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Test stats 🧪

Test	Results
Failed	0
Passed	2455
Skipped	216
Total	2671