If it is truly a metal roof there are several products that you might consider
Aluma-lock is an aspestous/asphault mix with aluminum in it (or was 50 years ago they may have removed the sspestous now) I do not recommend it personally but it has a high incidence of reflection and as I recall was sold for RV roof (But I've not sold it in what,,, 40 years)
Snow Roof, Quick-Cool and several other products are bursh on rubber roof coatings.. Now some are black (Do not recommend) and some are WHTIE,, Nice and bright WhITE.. Just like the roof on my RV they are WHITE... Friend used this type of product on her sticks and bricks and saw a major reduction in A/C cost.
(Of course all the FanDangles I did there helped that too.. Like 5 or 6 ceiling fans I dangled for her).... Got one more dangle to do but that's light, not fan).