I could be wrong... So experts please add on.
I think low-key-lighting pictures are composed with majority of dark areas but leave the subject properly exposed to show fine details. So I suppose need to spot meter at the selected area OR if using evaluative/matrix metering then need to compensate downwards and try until get it right. Thank god for digital!
High-key pictures are bright or white all round and leave the subject's darkest areas properly exposed to show details. So probably need to spot meter on selected area OR compensate upwards for evaluative/matrix metering.