Add a metric utility function to export ristretto cache stats

Created by: efritz

The precise code intel services have adopted ristretto for a high-performance in-memory LRU cache. This exports metrics so we can determine the number of hits and misses of the cache as well as its current cost.

I'm putting this in internal rather than in the bundle manager as it could be useful elsewhere (@keegancsmith suggested the cache in the first place, so I assume he has his eye on it).