What people want is that, if cgroup limits prevent a container from using more than M/N of CPU time (N the number of cores), then GOMAXPROCS defaults to M. Ditto other managed language runtimes and their equivalent parameters.
However, as far as I can tell, there's no clear way to figure out what M is, in the general case.
Again, I might be wrong as I did not use this directly in a couple years, but saying “the limit is 50% share of 10 cores” is not equivalent to “the limit is 5 cores”. This is still “how much” versus “how many”, and cannot translate into each other without sacrificing flexibility
GOMAXPROCS sets the number of live system threads used to run goroutines. The distinction between 50% of time on 10 cores and 100% of time on 5 cores doesn't really matter here: the recommendation is to set GOMAXPROCS=5 in both cases.
However, as far as I can tell, there's no clear way to figure out what M is, in the general case.