There may be cases in which a user wants to know which part of their code is unstable.
We use ObservedFuncs and UnstableCounters to print at exit which of the ObservedFunctions
are unstable under the -print_unstable_stats flag.
Patch by Kyungtak Woo (@kevinwkt).
but we need some assertion mechanism to make sure it's not possible to use -print_unstable_funcs without -handle_unstable.
Another idea I have: can we re-use PrintUnstableStats flag for that? I think it doesn't need handle_unstable. Let's just print functions under that flag as well.