prometheus-node-exporter-lua: fix missing conntrack values

If the /proc/sys/net/netfilter/nc_conntrack_* files are not present,
this exporter was outputting a blank value, which is invalid. These
files will not be present when using an image that doesn't include the
iptables and firewall packages (eg a minimal access-point type image).

This updates the collector to only output the metrics if the
corresponding /proc files are present.

Signed-off-by: Alex Tomlins <alex@tomlins.org.uk>

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

@@ -1,8 +1,12 @@
 local function scrape()
-  metric("node_nf_conntrack_entries", "gauge", nil,
-    string.sub(get_contents("/proc/sys/net/netfilter/nf_conntrack_count"), 1, -2))
-  metric("node_nf_conntrack_entries_limit", "gauge", nil,
-    string.sub(get_contents("/proc/sys/net/netfilter/nf_conntrack_max"), 1, -2))
+  local count = get_contents("/proc/sys/net/netfilter/nf_conntrack_count")
+  local max = get_contents("/proc/sys/net/netfilter/nf_conntrack_max")
+  if count ~= "" then
+    metric("node_nf_conntrack_entries", "gauge", nil, string.sub(count, 1, -2))
+  end
+  if max ~= "" then
+    metric("node_nf_conntrack_entries_limit", "gauge", nil, string.sub(max, 1, -2))
+  end
 end
 
 return { scrape = scrape }