Clarify / extend monitoring APIs to allow migration away from globals

[faec]

To support the Beats Receivers project, recent changes added a metrics namespace inside the top-level Info struct for each beat. This has been rapidly extended, most recently in the new input metrics API changes.

Unfortunately, there isn't a 1-1 relationship between the new Info-level metrics data and the various global registries used throughout the beats repository, so current uses are still more or less placeholders. We should more clearly define the registry/namespace hierarchies and their ownership/use, to allow a full migration away from process-level globals in the beats metrics, and replace them with an equivalent API that can enable metrics reporting in both "classic" beats and the new OTel beats receivers.

[elasticmachine]

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