no 1 or 2 glasses do not do much harm... if you are not very high and out of control diabetically... but if you haven´t gotten your numbers almost down to normal then I guess alcohol wil be another burden on your body..
1 glass can actually put your blood glucose a little down if it is not filled with sugar...I myself enjoy a tiny glass of single malt occasionally..
Metformin is cleared out of the body by your kidneys , I guess thats why they warn people ....but always ask your GP too, as he or she knows if you have other conditions that would all together talk against drinking alcohol