x-pack/filebeat/input/awss3 ; Fix nil hit panic when a getter is invoked on input metric

[bhapas]

What does this PR do?

This PR fixes the panic mentioned in #36077. If an input metrics is not initialised but when a Get is invoked on the metric to pull data , then a nil pointer panic is caused.

Why is it important?

To avoid nil pointer panics when metric getters are invoked

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.

How to test this PR locally

Without this PR , when go test -tags aws,integration -run TestInputRunSQS -v is run from x-pack/filebeat/input/awss3 then a nil pointer panic is caused and this PR fixes it.

Related issues

• Closes [filebeat][awss3][automation] Panic in integration test while polling input metrics for SQS #36077

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @bhapas? 🙏.
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]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[bhapas]

Pinging @elastic/obs-cloud-monitoring as a codeowner

[elasticmachine]

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Build stats

• Start Time: 2023-07-24T13:27:58.192+0000

• Duration: 135 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	6159
Skipped	357
Total	6516

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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[kaiyan-sheng]

This looks good to me but I'd like to get @andrewkroh 's approval on this before merging. 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-commented-aws-tests upstream/fix-commented-aws-tests
git merge upstream/main
git push upstream fix-commented-aws-tests