Absolutely no where with waitstaff, bartenders, or really anyone else. As a former waitress and bartender, finger snapping was the quickest way to be ignored. It is the lowest form of human interaction.
I was in a nice restaurant in DC and happened to see someone do this to a waitstaff. What makes you think that this is a good way to get their attention?
Trust me, “excuse me” or a “pardon me” works much better. And you’re much less likely to have anything “added” to your meal when it arrives.