Cherry-pick #26443 to 7.x: [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.x 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]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: Pull request #26628 opened

• Start Time: 2021-06-30T19:40:11.114+0000

• Duration: 79 min 50 sec

• Commit: 35146fc

Test stats 🧪

Test	Results
Failed	0
Passed	1949
Skipped	249
Total	2198

Trends 🧪

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	1949
Skipped	249
Total	2198