[CLOUD-151] Instrument encryption cache
Created by: rafax
Adds instrumentation to encryption/cache - should make it possible for us to decide if we need to tune its size (to reduce the number of KMS queries).
Testing
Enable encryption and encryption cache locally by adding
"encryption.keys": { "cacheSize":2048, "enableCache":true, "externalServiceKey": {"keyname": "external","type": "mounted","filepath": "/Users/rafal/dev/dev-private/enc.key"}},
to site config and observing metrics in Prometheus.