[Draft] x-pack/filebeat/input/{cel,httpjson}: fix handling of ADC metadata server credentials

[navnit-elastic]

Proposed commit message

x-pack/filebeat/input/{cel,httpjson}: fix handling of ADC (Application Default Credentials) 
metadata server credentials

HTTPJSON and CEL input do not handle ADC metadata server credentials correctly.
Currently they expects a JSON key in ADC which is not present when Filebeat is running
on Google Cloud Platform, as described in Google's SDK[1].

[1] https://pkg.go.dev/golang.org/x/oauth2/google#Credentials

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.

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Author's Checklist

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How to test this PR locally

Related issues

• Closes [Filebeat] HTTPJSON and CEL input encounters issues when attempting to utilize ADC (Application Default Credentials) in a GCP environment #44349

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @navnit-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-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[ShourieG]

@navnit-elastic, we should always split PR's for different inputs even if the change is relatively similar. We can cross reference the PR's but we should not club together.

[ShourieG]

We also need to check for empty string here. If it's empty then move to ADC flow

[navnit-elastic]

Got it

[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 44349-cel_httpjson upstream/44349-cel_httpjson
git merge upstream/main
git push upstream 44349-cel_httpjson

[navnit-elastic]

@navnit-elastic, we should always split PR's for different inputs even if the change is relatively similar. We can cross reference the PR's but we should not club together.

Agreed, I'll split this into two PRs.

[navnit-elastic]

Both PRs for CEL and HTTPJSON have been merged, so I am closing this PR.
HTTPJSON: #44436
CEL: #44571