[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

• 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.

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[mergify]

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

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)