feat: debug logging for cache metrics (#98)

*Issue #, if available:*
N/A

*Description of changes:*
- Add debug logging for cache metrics (hits, misses, hit rate, miss
rate). The logic is wrapped in `#[cfg(debug_assertions)]` which causes
it to be conditionally compiled only when the project is built in debug
mode to avoid performance overhead in release builds
- Example log statement: 
`METRICS: Cache HIT for secret 'secret2'. Total hits: 2. Total misses:
1. Hit rate: 50.00%. Miss rate: 50.00%.`

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under the terms of the Apache 2.0 license.

Author: ThirdEyeSqueegee