Weeping Cherry tree.
Cherry trees are susceptible to many leaf spot diseases. One is commonly called ‘shot hole’ because the infected tissue dries up and falls out causing the tiny holes you describe. This makes it look like an insect has been feeding on the leaves, but it's really the dead leaf tissue falling out.
Mild, wet spring and early summer weather promotes this leaf spot. Rake up and dispose of infected leaves that fall off the plants. The disease is not serious but causes cosmetic damage. Spraying with a fungicide is not practical as the spraying regimen begins when the new leaves emerge and continues every two weeks or so throughout the summer.
If you see anything new or different next year, send us new photos.