I'm on metformin for this reason. I asked my onco about it (he knows I will have read the research), and he said it's a good idea. My GP was happy to prescribe it - it's cheap, and by itself, it presents negligible risk of hypo to a non-diabetic. I have measured/graphed my blood glucose regularly for years (even before PCa), because I had a relative who was diabetic, but I'm not. However, I don't think metformin would require regular blood glucose monitoring if you aren't diabetic, but I would advise everyone on hormone therapy to check their blood glucose monthly using a personal tester, because HT can push you more towards diabetes (it was significantly raising mine, but still within normal range, although the metformin seems to have stopped the raise). If you are checking your blood glucose, you might find the HT has pushed you into the range where you can ask for metformin anyway - it is a recognised side effect, and the data may help convince your onco/GP that you know what you're talking about.
One significant factor is probably that both my consultant and GP treat me as an expert patient, and allow me a lot of control over my treatment, because I took the trouble to learn a lot about it from research papers during diagnosis. This means I can have good discussions with them, and have changed the course of my diagnosis and treatment as a consequence, but with their approval.
I would encourage all patients to learn as much about their conditions as they can - it can make a significant difference to your treatment. Not all clinicians like expert patients, but fortunately, my consultant, Macmillan nurses, and GP do. I try to do my part through support groups to educate other patients. As part of this, I will be running a second session at Mount Vernon Hospital on living on hormone therapy on January 15th - PM me if you would like to attend. (It won't have anything about metformin in it though.)