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

Start by tracing the cache manager's cache lookup and storage paths in the repository, since the issue names no files or tests. Determine where hit and miss events occur and how counts should be exposed for iOS and Android use; done means cache hit/miss counts are maintained during use and covered by the relevant tests.

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