I consider the libraries of the languages that allow the programmers to do everything (e.g. Python or JavaScript) are good at this: The library authors signal that these functions should not be called and these can be, but ultimately if you’re monkeypatching or something you can really do this. Obj-C is also a good language in doing this too…

Which metric? Leaky abstractions is a "Law", not a metric.

Are you looking for experience?

[ see Wikipedia for contextual meaning of law here, https://en.wikipedia.org/wiki/Law_(principle) ]

I think by "metric" he means your definition of a good abstraction, not the fact that all abstractions leak.