Update Product Variant Metafields

Updates a product variant *Metafield*. **Required Fields:** * none **Read-Only Fields** * id * These fields can only be modified by the app (API credentials) that created the metafield: * namespace * key * permission_set **Usage Notes** * Attempting to modify `namespace`, `key`, and `permission_set` fields using a client ID different from the one used to create those metafields will result in a 403 error message.