add helper script to extract timestamps from files

This is a pretty simple tool, but we've found this useful to check
some diagnostics. We've also found alternatives like:

https://github.com/Puppet-Finland/prometheus-file-age-metrics/blob/main/create-file-age-metrics.sh

... which gives you a metric like "out of date files", which didn't
like, because the threshold lives in the script instead of the
alerting layer (which we prefer).

The closest thing to this is the filestat exporter:

https://github.com/michael-doubez/filestat_exporter

... which provides way more metrics and seems overengineered for our
purpose.

path-timestamp.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+# Expose timestamp of given path
+#
+# Usage: add this to crontab:
+#
+# */5 * * * * prometheus path-timestamp.sh /var/lib/prometheus | sponge /var/lib/node_exporter/path-timestamp.prom
+#
+# Author: Antoine Beaupré <[email protected]>
+
+echo "# HELP node_path_modification_timestamp_seconds Last change timestamp"
+echo "# TYPE node_path_modification_timestamp_seconds gauge"
+for path in "$@"; do
+    printf 'node_path_modification_timestamp_seconds{path="%s"}' "$path"
+    stat -c %Y "$path"
+done