So C’ is where you add the metrics to C, and then you cherry pick that commit separately onto B and onto A, creating your 2 extra branches.
still need a way to know I should collect the metrics in these “detached” commits
If the metrics are data series (plots), you can use dvc plots diff A' B' C'. See plots diff. But this feature isn’t available for plain metrics yet 
— I just opened an issue for that.