Add storage metrics for local storages

[fmassot]

Currently, we register those metrics for object storage:

• object_storage_get_total
• object_storage_put_total
• object_storage_put_parts
• object_storage_download_num_bytes
• object_storage_upload_num_bytes

But we don't have any metrics for local disk storage. This is annoying as we have several users that are using quickwit with a local storage.

[guilload]

Also, local storage can be "not so local" with remote storage services (EBS, NFS, ...) mounted locally. It would be nice to add a label. Users could explicitly set the label in the storage config or we could infer it via introspection (device and mount point?).

We want those metrics for Airmail.