[0.9.0] Pool size metrics

What's Changed

Pool size metrics (using telemetry) were added. How to use them is described here: Working with metrics guide.

To enable pool size metrics, you need to set the pool_size_metrics parameter to true on the pool initialization:

children = [
  {Poolex, 
    pool_id: :worker_pool,
    worker_module: SomeWorker,
    workers_count: 5,
    pool_size_metrics: true}
]