Metadata Document Generator(Part of XRMToolbox) saved a lot of my time. I have exported in 30s all atributes (10k+) for selected entities like Account, Contact, Issue (Case), Lead, Opportunity from Dynamics 365 (Online). I believe it is the best possible way to extract metadata, document even complex CRM data model.
Unfortunately out of the box methods for that are not working properly, particularly when it comes to export bulk attributes!