> Transactions like full user flows start to finish, or 1 transaction = 1 post/get and 1 response?

For most applications we are talking closer to 1 transportation 1 web request. Distributed tracing across microservices is possible, the level of extra effort required depends on your stack. But that's also the out of the box, plug and play stuff. With lower level APIs you define your own transactions, when they start and end, which is needed for tracing applications where there isn't a built in framework integration (e.x not a web application).