if you need the meta data entries in your aggregate, then the handler interceptor is the best way to get it assigned to the message. If you needed the information in outgoing messages only (for example events published by the aggregate), then you should use the handler interceptor to assign correlation data to the current unit of work instead.
Another option to get the data in your aggregate is by calling the external service in your aggregate. The disadvantage of that approach, though, is that you fetch the information while holding a lock on the aggregate, blocking all other commands for that aggregate. That may, or may not be an issue. Conceptually, it’s a bit nicer, though.