[9.3](backport #49502) Metricbeat: Remove global path reference

[mergify]

Proposed commit message

Remove the global paths.Paths singleton fallback from BaseMetricSet.GetPath() in metricbeat.

GetPath() previously fell back to the global paths.Paths singleton when the instance's paths field was nil. All production callers already set paths via NewModule/newBaseMetricSets, so this only affects test code that constructs BaseMetricSet directly without paths.

An exhaustruct lint rule is added scoped to mb.BaseMetricSet to prevent future direct struct literal construction that omits required fields like paths. Existing construction sites are annotated with nolint:exhaustruct where justified.

This is part of the larger effort to eliminate global path state from the beats codebase.

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

# Run the linter to verify exhaustruct rule catches uninitialized BaseMetricSet
golangci-lint run --enable-only exhaustruct ./metricbeat/...

2. Run metricbeat integration tests

Related issues

• Closes [beatreceiver] remove global paths fallback in metricbeat BaseMetricSet.GetPath #49461

---

This is an automatic backport of pull request #49502 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 4666186 has failed:

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Changes to be committed:
	modified:   .golangci.yml
	modified:   metricbeat/mb/builders.go
	modified:   metricbeat/mb/lightmetricset_test.go
	modified:   metricbeat/mb/mb_test.go
	modified:   metricbeat/module/openmetrics/collector/collector_test.go
	modified:   metricbeat/module/prometheus/collector/collector_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   metricbeat/mb/mb.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

🤖 GitHub comments

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[elasticmachine]

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

[mergify]

This pull request has not been merged yet. Could you please review and merge it @orestisfl? 🙏