otlpmetricgrpc.New and otlpmetrichttp.New should return structs

[pellared]

All the metrics components SHOULD allow new methods to be added to existing components without introducing breaking changes.

otlpmetricgrpc.New and otlpmetrichttp.New are components returned as interfaces. It would not possible to add a new methods that would control the exporter without creating a new method like NewBis that would return a struct (or other interface). I think it would be more flexible if both methods return structs instead of Exporter interface. The same is already done for span exporters (see here).

Originally posted by @pellared in #3671 (comment)