It's actually possible, I just had to dig a bit deeper:

So, how did I do this?
In the HBoxContainer, Layout -> Container Sizing -> Horizontal should be set as "fill" (which is the default, I guess)
For the value label, I also had to go to Control -> Layout -> Container Sizing and check "Expand" to let it fill the remaining space of the BoxContainer.
If I wanted a fixed width for the label, maybe for styling purposes, I could've set that with Control -> Layout -> Custom Minimum Size, but then it wouldn't be much different from a GridContainer, as it seems.