Rename read/write_io.ops_per_sec to read/write.iops in rds metricset

[kaiyan-sheng]

What does this PR do?

This PR is to rename aws.rds.read_io.ops_per_sec to aws.rds.read.iops and rename aws.rds.write_io.ops_per_sec to aws.rds.write.iops in rds metricset. Also changing the field names used in dashboard/lens as well.

closes: #16079

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.

[mergify]

This pull request does not have a backport label. Could you fix it @kaiyan-sheng? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-10-15T19:07:10.404+0000

• Duration: 97 min 26 sec

• Commit: 7e472b2

Test stats 🧪

Test	Results
Failed	0
Passed	9508
Skipped	2537
Total	12045

💚 Flaky test report

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Pinging @elastic/integrations (Team:Integrations)

[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 rds_iops_rename upstream/rds_iops_rename
git merge upstream/master
git push upstream rds_iops_rename

[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 rds_iops_rename upstream/rds_iops_rename
git merge upstream/master
git push upstream rds_iops_rename