In terms of total people volumes, a company called shoppertrak has been providing technology to do this for at least 10 years.

This used a hidden camera, and the people appeared as fuzzy blobs; it was relatively easy to see how long individuals are dwelling at a particular shelf or display. Camera technology has definitely come a long way in the last 10 years, so there are probably many more features available today. I've been out of that sector since May 1997.