Clean up data layer after moving all plugins and interfaces to a common location

[elevran]

Several cleanup items discussed in #2228: moving data layer to a common package structure with other plugins.
Part of #2190.

• Metric names referenced by various plugins (e.g., here) should be defined with metrics type where they are produced
• optionally extranalize ValidateExtractorType()
• remove the concrete metrics and endpoint metadata types from the Endpoint interface and make them opaque Attributes

[elevran]

/assign

[ahg-g]

Thanks for doing that, I recommend waiting until #2161 merges so we don't cause conflicts with that PR

[nirrozenbaum]

/triage accepted