to_s:
size: size value
to_json:
first: first value
product.metafields.custom["size"] => size value
product.metafields.custom.size => size value

product.metafields.custom["first"] => first value
product.metafields.custom.first => first value

product.metafields.custom["last"] => last
product.metafields.custom.last => last

product.metafields.custom["to_h"] => to_h value
product.metafields.custom.to_h => to_h value

product.metafields.custom["to_liquid"] => to_liquid value
product.metafields.custom.to_liquid => to_liquid value