Migrate rds metricset to use cloudwatch input as light weight module

[kaiyan-sheng]

What does this PR do?

This PR is to migrate rds metricset to use cloudwatch input as light weight module. Two main parts in this PR:

In the original rds metricset, we are also making RDS DescribeDBInstances API call to get metadata for each RDS instance. This is added as a part of cloudwatch metricset now.
You can see under rds metricset, there is only a manifest.yml file which defines cloudwatch as the input.

We need to keep backward compatibility with metric names so the dashboard can still be used.
You can see rename processor is used in manifest.yml file under rds metricset to make sure with this PR we are not introducing breaking changes. Also please check the updated data.json file in rds metricset to see the renaming is effective.

Why is it important?

With this change, we no longer need to maintain the common code in both rds metricset and cloudwatch metricset.

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.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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• Build Cause: Started by user kaiyan-sheng

• Start Time: 2021-06-03T22:20:50.054+0000

• Duration: 105 min 12 sec

• Commit: 4b1671d

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Test	Results
Failed	0
Passed	8634
Skipped	2483
Total	11117

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Test stats 🧪

Test	Results
Failed	0
Passed	8634
Skipped	2483
Total	11117

[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 refactor_rds upstream/refactor_rds
git merge upstream/master
git push upstream refactor_rds

[ChrsMark]

👍🏼