Cherry-pick #26443 to 7.14: [metricbeat] fix aws cloudwatch metric tags when the number of resource is a multiple of 50 (#26385)

[kaiyan-sheng]

Cherry-pick of PR #26443 to 7.14 branch. Original message:

What does this PR do?

see issue #26385

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• 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.

Related issues

• Closes [MetricBeat] [AWS] if the number of resources is a multiple of 50, there is no tags field in the event #26385

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #26630 opened

• Start Time: 2021-06-30T19:49:57.381+0000

• Duration: 76 min 42 sec

• Commit: ea56fa1

Test stats 🧪

Test	Results
Failed	0
Passed	1949
Skipped	249
Total	2198

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Test stats 🧪

Test	Results
Failed	0
Passed	1949
Skipped	249
Total	2198