The OP was asking about Internet Traffic all of which is available to the router.
It still wouldnt be simple to monitor the traffic as the router would have to analyse it because, as you say, the traffic wouldnt come out of the LAN ports because of the switching.
Could always use an old PC to do the routing using one of the Linux packages and use the current router in bridge route?
Ex <n>ildram , been to SKY MAX - 15,225 Download
BE Unlimited - 21,000 Download 1,200 Upload,
Moved house, now BE Unlimited 6,500 Down, 1Mb/s up - gutted!
FTTC Cab installation commenced 12th April - expect full 80 / 20 - bye bye BE, hello BT Infinity soon!