You cab do this the easy and simply use zero for the cost basis of the fractional share sold since the amount is so low. That preserves all your cost basis for the full shares received in the spin-off.
For future sales , your cost per Brighthouse share would then be the following:
Use the dollar cost basis of all your MetLife stock and multiply it by .103635%. That's the portion of your cost allocated to Brighthouse. Divide that number by the total number of Brighthouse shares received to arrive at the cost per Brighthouse share.