I implemented this functionality where we work, but we did it properly. We haven't done the "viewing" one, but we track every "add to cart" and every actual tr

I implemented this functionality where we work, but we did it properly. We haven't done the "viewing" one, but we track every "add to cart" and every actual transaction, store them in a backend database and then use this to show badging that simply says "popular" on the top 5-10% of products in a given category, which is based on interactions over the past 24h. Sure, we could improve it and make it more accurate etc, but I feel comfortable that we did things reasonably properly and it is a true and honest representation of the popular products. There are SAAS providers that sell this functionality and it's super expensive ($tens to $hundreds of thousands annually) which is an absolute joke compared to how trivial it is to implement.