Cache memory limit with `weigher` function

[jiangzhe]

My use case is to limit cache memory usage.
I found an old issue related to this topic: #201
In that thread, an approach of weigher function with the formula size_of(key) + size_of(value) + overhead is proposed.
I wonder if this information is still up-to-date and how to calculate overhead based on current codebase?
I think it would be very useful to put this common use case in document and add some helper functions (overhead on different setting). I'd be happy to help if author has no bandwidth on it.