The async instruments currently return an error if and only if there are
no aggregators returned from a resolve. Returning no aggregators means
the instrument aggregation is drop. Do not include this in the error
reporting decision.

The instruments passed to RegisterCallback need to have some aggregation
defined otherwise it is implied they have a Drop aggregation. Check that
at least one instrument passed has an aggregation other than Drop before
registering the callback with the pipelines.

Also, return an error if the user passed another API implementation of
an asynchronous instrument.