Metrics on K/V paths

#### Feature Description

It would be great if we could have custom counters on K/V paths emitted. Even "x keys added since last flush" would be great.

#### Use Case(s)

If I have a lot of activity in my Vault, and I want to have a rough idea of what secret engine is getting the most activity, or I want to see if I have an abnormal number of leases being created. Even the anonymized data from knowing how many keys were added vault/auth/abcd1234-abcd1234-abcd1234-abcd1234 over a given period of time would be great.

I have no issues doing the actual summation of data in my graphing, just having the data available would be awesome. Right now it's required to do a recursive key lookup which is extremely heavy on Consul and I'd like to avoid it.