Cache hit/miss metrics

## 🏗 Enhancement Proposal

Have cache manager keep a count of cache hit/misses during use.

### Pitch

Something that popped into my head while testing for #179, it would be useful for the cache manager to store cache hit/miss counts. This would allow app developers to better understand how the cache is performing, and adjust usage to improve cache hit rates.

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android