How is a "true monotonic counter" supposed to interact with atomicity and isolation of transactions? Sounds like a global bottleneck. Is uniqueness sufficient

How is a "true monotonic counter" supposed to interact with atomicity and isolation of transactions? Sounds like a global bottleneck. Is uniqueness sufficient or is it really required to step through all integers without missing any? EDIT: Missing primary keys in-between also bothers me, however, never found a decent solution for this at scale.