Split request and hit metrics into separate metrics, rather than using
labels. This avoids duplication of data and makes metric math easier.
* Count cache errors separately to avoid weird math.
* Hit ratio: `registry_storage_cache_hits_total / registry_storage_cache_requests_total`
* Miss ratio: `1 - (registry_storage_cache_hits_total / registry_storage_cache_requests_total`
* Misses: `registry_storage_cache_requests_total -
registry_storage_cache_hits_total`
Signed-off-by: Ben Kochie <superq@gmail.com>