[metricbeat/graphite] Fix out-of-bounds panic in graphite server metric processing

[shmsr]

Proposed commit message

Fixes a potential panic in the graphite server metricset when processing metrics with fewer parts than the configured template expects.

The template.Apply() function iterated over template parts and accessed parts[i] without verifying that the index was within bounds. When a metric had fewer segments than the template, this caused an index out of range panic. Now the patch handles this by adding a bounds check at the start of the loop iteration.

Checklist

• [x] My code follows the style guidelines of this project
• [x] 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.
• [x] I have added an entry in ./changelog/fragments using the changelog tool.

[github-actions[bot]]

:robot: GitHub comments

Just comment with:

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

[mergify[bot]]

This pull request does not have a backport label. If this is a bug or security fix, could you label this PR @shmsr? 🙏. 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/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[github-actions[bot]]

@mergifyio backport 8.19 9.1 9.2 9.3

[mergify[bot]]

backport 8.19 9.1 9.2 9.3

❌ No backport have been created

• #48130 [8.19](backport #47916) [metricbeat/graphite] Fix out-of-bounds panic in graphite server metric processing has been created for branch 8.19
• #48131 [9.1](backport #47916) [metricbeat/graphite] Fix out-of-bounds panic in graphite server metric processing has been created for branch 9.1
• #48132 [9.2](backport #47916) [metricbeat/graphite] Fix out-of-bounds panic in graphite server metric processing has been created for branch 9.2
• Backport to branch 9.3 failed

GitHub error: Branch not found