what can be the proxy variable for measure of product innovation?
UN COMTRADE has export data by country-product at the HS 6-digit level. What you could do to find out when the product was first exported is to get data for a country's export for product, say, with HS code 123456, from as far back as possible, and see when was the first year that the export of this product is not zero.

I've seen number of patents filed used as a proxy for product innovation in some papers. That might work for your paper as well?