Introduce Irmin stats for R/W in Prometheus reporter

Context

Irmin exposes some statistics to have an overview of what happens. This PR intends to expose those metrics through the Prometheus interface provided by a tezos-node`.

Manually testing the MR

This MR can be checked by running a node with Prometheus turned on:

tezos-node run --metrics-addr=:9091

and then check at localhost:9091.

Checklist

• Document the interface of any function added or modified (see the [coding guidelines](https://tezos.gitlab
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR