well for now the return type is just `Map<String,String>` so we could have a common interface.  if the intention is specific return types we should mark the methods `@Beta`.

is `getUsageMetadata` a better name for the method?  `toMetadataRecord` feels slightly wrong tho maybe i'm coming from the wrong pov.

if so common interface could be `HasUsageMetadata` -- or else fine putting on `EntityInternal` with javadoc saying empty for entities, and marking `@Beta` with note return type might become more specific.