prometheus_folder_size_exporter

This version adds the support for asynchronous folder size calculation. This is useful if the standard, blocking calculation takes too long (because perhaps the storage is slow or the number of folders too big).

Just specify the -b option with the number of seconds between calculations.

Note: the first calculation will always be synchronous in order to avoid returning invalid data.

• Added user label. Thank you scatenag for your first PR (https://github.com/MindFlavor/prometheus_folder_size_exporter/pull/1)!