make including prometheus optional

[leehinman]

Describe the enhancement:

We had to do remove beat receivers from elastic agent because the prometheus imports conflicted. We need to figure out a way to import beats without prometheus until such a time as the import conflict can be resolved.

Describe a specific use case for the enhancement or feature:

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[leehinman]

Fixed by @khushijain21 in elastic/elastic-agent#7686