You can use export to export your metafields related to products or variants. Since the application does not store the metafields in the application (all metafields are stored via Shopify API so they are actually stored by Shopify), the application would first fetch all your products or variants and then iterate through each product or variant to find any related metafields. This process can take a while depending on the number of products or variants in your Shopify store.
The exports are place in a queue and processed. Since there is any timeout (it happens frequently with Shopify API access), the export can failed but be placed back into a retry queue. It is not usual for a export to take days. Keep refreshing the export listing page to see status of an export. If you think an export has stalled (the progress count does not update), please contact email@example.com for assistance