I am also not a native speaker, but having a stream/river in mind, then
upstream seems to be where the water comes from and downstream where the
water is going to.
However, the OP mentioned that one micro service called the other which
tells me something about how the connection is established,
I've seen both usages depending on the writers' perspectives. For that
reason I avoid the terms upstream and downstream in this context (services)
like the plague and always ask people for clarification without using these
two words. It's always fun to see this then sparking totally surprised
