Vault creates an LRU cache that is used when interacting with the
physical backend. Add telemetry when the cache is hit, missed, written
to and deleted from. Use the MetricSink from ClusterMetrics
* Port over some SP v2 bits
Specifically:
* Add too-large handling to Physical (Consul only for now)
* Contextify some identity funcs
* Update SP protos
* Add size limiting to inmem storage