Add monitoring capabilities to the pool

## Overview
Our awesome pool is heavily used in `cloud-deployer` and it's super fast!
We would like to send dynamic slack notifications during the workers' processing like how many items have been processed so far, how many pending, errors etc.

## Possible Implementation
I'd consider two different implementations

#### Add API so that consumers call it to get the information needed
This gives consumers the flexibility of poking the API directly and processing the results as required. Ideally, I'd expect that results are the structure that has been passed in workers during pool initialization

#### Inject a function inside the pool
This would require the addition of 2-3 new fields during pool creation
- Function to inject
- Frequency
- Initial timeout before injecting the function for the first time

The pool would know then when to run this function internally
This gives less flexibility to the consumers but hides all the details inside the pool

## Context
Make our awesome pool more awesome