So I was thinking while in the middle of one of my chains, it sure would be nice to know how many mobs I have killed in my current chain, maybe a small number somewhere we can position on the screen to keep up with our current kills etc.

When the mob dies it'll tell you, on his head will appear a floating number that stays for a sec or two. Good enough imho.

I have never noticed it. I am too busy usually looking at other stuff. Is it white text that looks like the combat spam?

Floats up at the same sort of time as the exp notification, same colour

Note it doesn't appear if your video settings are set to low, only medium and high.