prometheus-node-exporter-lua: Add support for node_filesystem_* metrics

[wjam]

Maintainer: @champtar
Compile tested: Lua only, so not compilable
Run tested: TP-Link Archer C7 v2, OpenWrt v23.05 - I've been running the Lua code for a while with no issues and the publicly available Grafana dashboard (https://grafana.com/grafana/dashboards/1860-node-exporter-full/) has been happy with the data

Description:

Add a collector for the various fileystem metrics which matches the node-exporter behaviour. This collector supports the following metrics:

• node_filesystem_size_bytes
• node_filesystem_free_bytes
• node_filesystem_avail_bytes
• node_filesystem_files
• node_filesystem_files_free
• node_filesystem_readonly

[wjam]

Minor comments for @champtar to review:

• I went with filesytem rather than disk or disk_use which was mentioned in prometheus-node-exporter-lua: Disk space metric #14358 as it matched the node-exporter collector name.
• As commented in the code, / is excluded as it's an overlay filesystem which does mean the linked Grafana dashboard has a widget showing N/A - which is the same behaviour as node-exporter.