prometheus-node-exporter-lua: Add "node_textfile_mtime_seconds" metric

…for textfile collector, to make it more consistent with the upstream
Prometheus node-exporter

Signed-off-by: Rob Hoelz <rob@hoelz.ro>
[bump version]
Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
(cherry picked from commit 4edae84499)

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

@@ -3,8 +3,14 @@
 local fs = require "nixio.fs"
 
 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')
+    local stat = fs.stat(metrics)
+    if stat then
+      mtime_metric({ file = metrics }, stat.mtime)
+    end
   end
 end