feat: add metric pgrst_jwt_cache_size in admin server

[taimoorzaeem]

Add metric pgrst_jwt_cache_size in admin server which shows the cache size in bytes.

[steve-chavez]

Don't forget the feedback about the actual size in bytes #3801 (comment)

Ultimately, I believe we're going to need an LRU cache for the JWT cache to be production ready. So having this metric in bytes will be useful now and later.

[taimoorzaeem]

Don't forget the feedback about the actual size in bytes #3801 (comment)

Calculating "actual" byte size of cache is sort of tricky (haskell laziness is a lot to deal with sometimes) and I still haven't figured it out YET. In the meantime, I have written some code to approximate the cache size in bytes.

It works as follows:

Data.Cache gives a function toList which returns that cache entries in a list of tuples ([(ByteString, AuthResult, Maybe TimeSpec)] in our case).

Now, we can use ghc-datasize library code to calculate the byte size of ByteString and AuthResult but not Maybe TimeSpec (because recursiveSizeNF only works on types that are an instance of NFData typeclass, hence I am calling it an "approximation").