[mdatagen] Documentation is not generated for resource attributes without metrics

[crobert-1]

Describe the bug

When looking into open-telemetry/opentelemetry-collector-contrib#34210, I was confused as to why no documentation.md file was generated for the EKS detector of the resource detection processor, as it has a metadata.yaml file. However, I found that the documentation.md file is only generated if there are metrics or internal metrics defined.

Steps to reproduce

Run mdatagen against a metadata.yaml file that does not include metrics or internal metrics, but includes resource attributes

What did you expect to see?

A documentation.md file that includes information about valid resource attributes and how to enable and disable them.

What did you see instead?

No file is generated.

What version did you use?

v0.104.0

[ChrsMark]

@crobert-1 I tried my hand at it and filed #10705. I'm not quite familiar with mdatagen concept so I'm not sure if I have missed/broken anything in my attempt :).