I'm not sure what is common in the field these days, so I'm definitely interested in hearing other responses to this query.  But there are a number of possibilities that I have come across that you might consider. It mostly depends on what you mean by "interaction," what kind of space it's in, and why you are tracking this information.  Most people who come into contact with public art don't interact meaningfully with the artwork and may not even notice it depending on what the space is.  

The QR code analytics have the advantage of definitely resulting from an interaction with the artwork, though most people don't scan the QR codes even if they photograph the artwork, so you are missing a lot of interactions.  You could do some social media scraping if you have someone with coding skills to see how many people post photos of the art, but that doesn't measure more ephemeral interactions.

The other options are related to how people measure activity in public space.  There are pedestrian counters that can sense how many people come by and maybe walk up to the artwork if it is out of the normal path.  Other measurement strategies rely on observation and survey methods and these are probably the best, though obviously very resource intensive. There are measurement strategies based on traffic numbers if the artwork is placed in a way that is visible to cars, but that's a very small number of works.

If the artwork is placed on the grounds of a public facility (park with recreation staff, library, etc), they probably have methods of reporting how many people come to their facility, with varying levels of reliability.  

I'm curious to see if anyone has tried anything else I'm not thinking of!

Best,

Susannah