Feature/cache metrics

Adds cache metrics via active_support AS:N instrumentation:

    rails.cache.read
    rails.cache.read.time
    rails.cache.write
    rails.cache.write.time
    rails.cache.fetch_hit
    rails.cache.fetch_hit.time
    rails.cache.generate
    rails.cache.generate.time
    rails.cache.delete
    rails.cache.delete.time