I can't answer the question re a metallic taste (although I have heard that is a pg symptom which I never had). I just thought I'd add my story.

I have PCOS & I conceived both my babies on Metformin. I too started on a low dose (1/2 tab) and increased by 1/2 a tab every three weeks. When I got to 2 & 1/2 tabs, I started having side effects (mainly nausea) so dropped back to 2 tabs.

My cycles went from 90+ days to about 40 days with DS & 35 days with DD. I lost a couple kgs with DS too but I didn't lose anything with DD. I was focusing a lot more on eating low GI with DS though. It took 6 cycles (9 months) to conceive DS & 5 cycles (7 months) to conceive DD. I started taking the tabs prior to TTC DD so I would be on the full dose when we started.

Good luck with your cycle N2L