fix: ensure metric emission even when slice attr is empty

[StarpTech]

Motivation and Context

The metric store accepts slice of attributes as a first argument. Those attributes will result in multiple metric emissions to produce correct metric series in prometheus. The problem was when we passed an empty slice. This resulted in no metric emission at all.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/cosmo-docs.
• I have read the Contributors Guide.

[df-wg]