Avoid use of functor-level state for tracking statistics

[tomjridge]

Stats record is supposed to be per-index, but currently it is a global variable; presumably we should change this

[craigfe]

Similar to mirage/irmin#1528.

Perhaps @maiste's work on Grafana & exposing these metrics will provide some impetus for improving this.

[maiste]

Unfortunately, my current implementation is Tezos specific and work on top of the actual implementation.

I agree with you, but I currently have no good suggestion to solve the problem.