Add Stats API

[Yiling-J]

• Add RecordStats option to builder
• Add entry_count/get_count/miss_count/evict_count stats(total/ram/nvm), atomic uint64 should be enough
• Add nvm read/sync time percentile stats and loading cache load stats, using sliding window(fixed size, 60 seconds, 60 digests) & t-digest
• Add Stats API

[aryehlev]

@Yiling-J love this repo I've been wanting to do this for a while,
I can take this issue on me if u want

[Yiling-J]

Hi @aryehlev 1,2,4 in my comment are not difficult to implement, but the third one is a little tricky. I've done some work on the branch: https://github.com/Yiling-J/theine-go/tree/feature/stats, but the code is a bit messy. I'm busy recently so contributions are welcome! If you're interested in taking this issue, you can take a look at my branch for some ideas.

[Yiling-J]

Hi @aryehlev, are you working on this? I will have some time next month, if you haven't started yet I will try to finish it.