filebeat: Fix scenarios for short message and invalid month number in syslog messages

[bhapas]

Proposed commit message

The syslog messages , if they do not contain a month or contain an invalid month number and if they contain short message than 3 characters , the input errors out.

This PR fixes this error.

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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

How to test this PR locally

Unit tests are added to test this feature.

go test -run TestParserRFC3164InvalidMonthNumeric ./filebeat/input/syslog
go test -run TestParserRFC3164ShortMessage ./filebeat/input/syslog

[github-actions]

🤖 GitHub comments

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 @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-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.

[elasticmachine]

Pinging @elastic/integration-experience (Team:Integration-Experience)

[github-actions]

@Mergifyio backport 8.19 9.1 9.2

[mergify]

backport 8.19 9.1 9.2

✅ Backports have been created

Details

• #48037 [8.19](backport #48031) filebeat: Fix scenarios for short message and invalid month number in syslog messages has been created for branch 8.19
• #48036 [9.1](backport #48031) filebeat: Fix scenarios for short message and invalid month number in syslog messages has been created for branch 9.1
• #48038 [9.2](backport #48031) filebeat: Fix scenarios for short message and invalid month number in syslog messages has been created for branch 9.2