Just a quick note on the combination in the headline. I wanted an affordable, light (something that I could hang around my neck and walk around with) and long telephoto lens with excellent image quality. The best lens I could find was the Canon 300mm f4 IS. I got one in excellent condition for $800 and then a Metabones adapter for it (around $400). All in all a pretty affordable, sharp and light telephoto kit for the Sony A7. The Canon 300mm/f4 was on the list of supported lenses for the adapter.
So I got quite upset when I received the adapter and put it all together, only to discover that the autofocus did not work. It just hunted and hunted, and settled on blur 99% of the time. I googled and googled for solutions or other people who had similar problems, but couldn't find any useful information. I was on the verge on buying a A7ii, in the hopes of its phase detect AF option in the menu could fix the issues. But then I stumbled upon the solution. Apparantly the Metabones adapter has two modes (hard to know since it is not shipped with any kind of manual or instruction leaflet). It has a green mode and an advanced mode. And I needed to switch my Metabones adapter to the advanced mode to get my lens to work. In advanced mode the lens works very well and the AF is decently fast.
I just wrote this down here on my blog in case any other lost soul is scavenging the forums and internets for a solution to this problem. Documentation for the modes and how to switch between them is available here.