metal can be used as a resistance without any extra option.
But the impedance is too small that we usually use other types of resistances.
The metal resistance is parasitical when it is used as a interconnection line.
i searched in the documentation of the umc pdk; it tells that the only layer used is metal 1 , it seems that it requires no additional mask, you can simply put it in your schematic for certain value of resistance , then take the dimensions from layout and place a metal with the same dimensions , the run parasitic extraction and see the resistance wether same or differ