I wouldn't bother with measurement. None of the devices there, even in multiples, will even remotely exert a power outlet. As a ballpark, monitors and similar sized objects will rarely draw more than 100W, while handheld sized objects like the Pi will draw around 15W worst case. You're not gonna hit the outlet's limit unless you try really really hard.
You only need to start paying attention once you plug in either:
- Large electrical appliances that deal with heating or moving parts (washing machine, space heaters, aircon, microwave)
- Beefy desktop PCs (e.g. with multiple GPUs or multiple PCs)