prometheus-node-exporter-lua: expose node_os_info

prometheus-node-exporter-lua: expose node_os_info

the original node_exporter exposes a node_os_info metric with a set of
data about the system [1] which is then used by several dashboards.

openwrt.lua already exposes OS info, but using the node_openwrt_info
metric requires changes to existing dashboards, and would require more
complex lookups when there are non-OpenWrt hosts in the overview too.

as we've already called ubus and fetched the data, we can expose it in
two formats easily.

[1] https://github.com/prometheus/node_exporter/blob/d6d0e710bb7daf07a2743fde060f0d5f32c565f3/collector/os_release.go#L190-L192

Signed-off-by: Evgeni Golov <redacted>