prometheus-node-exporter-lua: Fix broken textfile collector

A recent change (00d420e80) removed the `output` function, which textfile.lua was
using to output metrics.  So use the underlying output handle instead

Signed-off-by: Rob Hoelz <rob@hoelz.ro>

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

@@ -4,7 +4,8 @@ local function scrape()
   local mtime_metric = metric("node_textfile_mtime_seconds", "gauge")
 
   for metrics in fs.glob("/var/prometheus/*.prom") do
-    output(get_contents(metrics), '\n')
+    out:write(get_contents(metrics))
+    out:write('\n')
     local stat = fs.stat(metrics)
     if stat then
       mtime_metric({ file = metrics }, stat.mtime)