Supplements?

I don't take any vitamin or mineral supplements because I don't have any deficiency symptoms. If I were to suffer symptoms I would ask my GP for the relevant blood tests and then consider matters. But that is me. Other people may disagree.
One exception to that is salt (sodium) because on a low carb diet we do need to up our salt intake, which I do by adding some to my food. As you are on Metformin you may like to investigate vitamin B12 as Metformin can compromise this. Magnesium and potassium are also other minerals that low carbing may affect.

Have a Google for the deficiency symptoms of nutrients you are concerned about and see if any apply to you.

As a matter of interest, I wouldn't trust what MFP says about sodium, calcium, iron, cholesterol etc. as these are only as good as the person who entered the data in the first place. When I was using it, I noticed many of the foods (probably the majority) either had no entries for these, or the measurement units entered were wrong - remembering that MFP members are from all over the world, and other countries have different food labelling rules, and may use different measurement units. The only way to be sure is to calculate these levels yourself and that is arduous. Also, the cholesterol from food is insignificant in raising our body's cholesterol levels. (Our livers make most of it)

I agree that unless you've had tests to confirm you are deficient in any area(s), as long as you're eating a varied and healthy low carb diet, you probably have no need to worry.

The only thing I've felt the need to watch is potassium, as if I lose too much fluid, I know from previous experience with taking diuretics that I'm liable to get leg cramps, so I just make sure I'm eating plenty of vegetables rich in this - in place of my previous go-to bananas.

Just out of idle curiosity, because I've often wondered about this having seen other people express similar concerns: do you think that low carbing is less adequate than you previous diet in providing all the correct nutritional content for you?

Robbity