Dear @OlgaP,
I had not actually heard of using metformin as an anti-HBV drug, so I did a little bit of research.
Looks like there is an interesting study showing that metformin does decrease HBsAg, HBeAg and HBV DNA levels in a petri dish: https://onlinelibrary.wiley.com/doi/abs/10.1111/jvh.12187. I think this has justified a clinical trial that is ongoing in China: https://clinicaltrials.gov/ct2/show/NCT04182321.
I do not think however it will have a “huge overall positive impact”. A paper looking at HCC recurrence showed no effect at all of metformin in preventing it: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0247231. Also, there was a huge study of more than 700,000 people which found no effect of metformin on liver cancer rates in HBV patients: https://journals.lww.com/md-journal/Fulltext/2015/02020/Cancer_Risk_in_HBV_Patients_With_Statin_and.5.aspx (there is a slight protective effect from statins, and a little more if you combine statins with metformin).
Cheers,
Thomas