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 <evgeni@golov.de>

utils/prometheus-node-exporter-lua/files/usr/lib/lua/prometheus-collectors/openwrt.lua

@@ -12,13 +12,22 @@ local labels = {
     target = b.release.target
 }
 
+local os_info = {
+    id = string.lower(b.release.distribution),
+    name = b.release.distribution,
+    pretty_name = b.release.distribution .. " " .. b.release.version,
+    version = b.release.version,
+    version_id = b.release.version,
+    build_id = b.release.revision,
+}
+
 local b = nil
 local u = nil
 local ubus = nil
 
 local function scrape()
     metric("node_openwrt_info", "gauge", labels, 1)
+    metric("node_os_info", "gauge", os_info, 1)
 end
 
 return { scrape = scrape }
-