Making aggregators interfaces to support different flavors

[rakyll]

In the metric package, we provide different flavors of the aggregator types, such as Float64SumObserver
vs BoundFloat64UpDownCounter. This is adding some mental overhead for the developer and requiring them to see another type. Instead of exporting various concrete types, would it be better to turn Float64SumObserver into an interface and provide two unexported concrete types?

[rakyll]

Or following this model at #1292, we split the bound types into separate packages, so they don't confuse the users.

[jmacd]

Either approach sounds good.

[MrAlias]

This issues looks to have been resolved by #2587