x-pack/filebeat/input/internal: split httpmon out of httplog

[efd6]

Proposed commit message

This moves the metric monitoring code out into its own package as metrics aren't logs.

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• 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.

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• Start Time: 2023-08-28T09:42:30.155+0000

• Duration: 76 min 12 sec

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Test	Results
Failed	0
Passed	3132
Skipped	176
Total	3308

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[ShourieG]

LGTM

[belimawr]

@efd6 could you also add an entry in the CHANGELOG-developer.next.asciidoc?