Looks like it might be flea beetle damage. They are a common pest of many different plants.
"Flea beetles produce a characteristic injury known as "shotholing." The adults chew many small holes or pits in the leaves, which make them look as if they have been damaged by fine buckshot. Young plants and seedlings are particularly susceptible. Growth may be seriously retarded and plants even killed. " https://www.ext.colostate.edu/pubs/insect/05592.html
They are called flea beetles because they jump like fleas. If you shake the plants a little, you will probably see them jumping.
"Floating row covers or other screening can exclude the beetles during seedling establishment. In isolated plantings, thick mulches may also help reduce the number of flea beetles by interfering with activity of the root and soil stages. Flea beetles can be vacuumed off foliage, but this practice must be repeated frequently. ...Diatomaceous earth is one of the more effective repellents, applied as a dry powder to the plants. Horticultural oils and some neem insecticides also have some repellent effect on this insect." same source
Chinese mustard, daikon, radish work well as trap crops for them, something that they prefer to your cucumbers.