If I can butt in here....my copy of Bilous says words to the effect that the mechanism by which metformin works "...is not fully understood" .
Edit - back home now. Bilous says "Metformin is a derivative of guanidine, the active ingredient of goat's rue (galega officianalis) used as a treatment for diabetes in mediaeval Europe. Metformin increases insulin action (the exact mechanism is unclear) lowering glucose mainly by decreasing hepatic glucose output." pg 82-3 4th edition.
In other words it works, but they don't know how. Bit like magnetism.
One of the most entertaining threads on here in a while. Goats, hurricanes, kittens, metformin. Got it all. If the goats were Tennessee Fainting Goats it would be perfect.