Fix a couple of bugs in the logic for how AWS metric periods are calculated

[tommyers-elastic]

Firstly, we clarify that periods are always whole-minute durations.
Any period that is less than or in between minutes is rounded up to the
next whole-minute.

Secondly, we ensure that the resulting time period is always in the past.
This stops us getting empty metrics for the current-minute period.

Thirdly, we follow the AWS guidelines of aligning periods to whole
multiples within the hour e.g. 10:25->10:30 instead of 10:27->10:32
for a 5 minute period.

Fixes #32687
Fixes #32640

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.
If this is a bug or security fix, could you label this PR @tommyers-elastic? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)
  To fixup this pull request, you need to add the backport labels for the needed
  branches, such as:
• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2022-08-25T15:15:29.125+0000

• Duration: 84 min 27 sec

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[kaiyan-sheng]

Overall the change looks good to me. One question: adding now as an input parameter for GetStartTimeEndTime, is it to make the tests easier?

[tommyers-elastic]

Overall the change looks good to me. One question: adding now as an input parameter for GetStartTimeEndTime, is it to make the tests easier?

yep!

[kaiyan-sheng]

Could you add a changelog for this one please? Thanks!!

[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 fix-cloudwatch-metric-interval-calculation upstream/fix-cloudwatch-metric-interval-calculation
git merge upstream/main
git push upstream fix-cloudwatch-metric-interval-calculation