[9.2](backport #46844) [beatreceivers] fix race condition in beat LoadMeta

[mergify]

Proposed commit message

Fix race condition in beat LoadMeta function. If 2 (or more) beats had the same path to the meta file, then it was possible for one not to start. This would only affect beat receivers because a normal beat requires a unique path to the meta file.

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.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

cd libbeat/cmd/instance
go test . -v -count 1 -run TestMultipleLoadMeta

Related issues

• Closes [beat] LoadMeta has a race condition. #46841

Use cases

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Logs

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This is an automatic backport of pull request #46844 done by [Mergify](https://mergify.com).

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