To record the traffic is very simple. BUT, and that's a big but for a few things to keep in mind:
- Creating reports based on your traffic log will be a lot more complicated in comparison
- Your server needs to be very fast and can keep up with the traffic otherwise the websites you are tracking will appear to load slower and the owners will not be very happy
- Depending on how many and how busy the websites you are tracking, you may need a server or a cluster of servers with quite big storage and processing power
If you are thinking about doing this for commercial purpose, there's already a few pretty big players in this space so do some research before making big commitments.