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.
Oh Lord, won’t you burn me a Knoppix CD ?
My friends all rate Windows, I must disagree.
Your powers of persuasion will set them all free,
So oh Lord, won’t you burn me a Knoppix CD ?
(Missquoting Janis Joplin)