Judging from your comments the situation in USA must be very different
from in the UK.
The vast majority of people here have a router-modem, supplied with a bb
package from an ISP. My guesstimate would be that this cover well over
99% of users. Many are left with several units after switching ISPs.
I talked with a fellow from Australia a while back and he reported the same
thing for his area. Lots of DSL, very little cable. Lots of combo units,
very few standalone units. You're right, the combo units are relatively
scarce here in the States, fortunately. Given a choice, I would never take a
combo unit. If forced to take a combo unit, I would try to make it
transparent, (bridge mode), so that I could use my own router.
It falls rather reluctantly on UK ears to be advised to purchase another
router for the express purpose of being able to monitor traffic through
the already installed one; as well as then having to install different
firmware in the new one.
No worries. You asked how to measure all of that disparate traffic on your
Internet connection and I think I gave you the best solution. That's all I
can do. To me, there's something very appealing about having full control
over my networking gear.