Metformin

A metallic taste in your mouth is a listed side effect of Metformin unfortunately. Have you been on it long?
The suggestion on the NHS website is to talk to your GP if it carries on and bothers you:
https://www.nhs.uk/medicines/metformin/
Have a look at the paragraph entitled side effects. Paragraph no. 5

It doesn't last long and is only a side effect of the drug start up.. It will go away in a few weeks to a month once your body gets used to it... Although don't just put it down to the Drug, as Covid has also been known to produce that symptom... Best contact your GP to be safe than sorry and do an LFT test

I have had this side effect a few times when I have not felt 100% well (non T2 related). Hope you get your medication reviewed and you don't experience the metallic taste.