Is it known what the mechanism is for metformin's effect on longevity?

If it's due to its ability to reduce blood glucose levels, then berberine may be another way to get a similar effect.

Berberine stimulates glucose transport through a mechanism distinct from insulin

https://www.sciencedirect.com/science/article/pii/S002604950...

Efficacy of Berberine in Patients with Type 2 Diabetes

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2410097/

It also lowers cholesterol

Berberine is a novel cholesterol-lowering drug working through a unique mechanism distinct from statins

https://www.nature.com/articles/nm1135

But pure berberine is not absorbed very well in the intestines, so you need to buy a good formation for it.

Enhancement of sodium caprate on intestine absorption and antidiabetic action of berberine

https://www.ncbi.nlm.nih.gov/pubmed/20237966/

Amorphous solid dispersion of berberine with absorption enhancer demonstrates a remarkable hypoglycemic effect via improving its bioavailability

https://pubmed.ncbi.nlm.nih.gov/24607213/

The best thing might be to cook with barberries:

https://thecaspianchef.com/2020/01/18/zereshk-polo-saffron-b...