add metricbeat receiver

Proposed commit message

This adds a metricbeat OTel receiver. This is necessary to run beats as OTel receivers.

[x] 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
[x] 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.

None. You have to import beats into your OTel project and instantiate a

cd x-pack/metricbeat/mbreceiver
go test -v .