Discard metrics output. In this instance we only want debug

That'll make it easier to identify what are the obsolete packages in the
output. The openmetrics output is not needed during investigation.

howto/upgrades.md

@@ -129,7 +129,7 @@ following two commands:
     apt list "?obsolete"
     # The following will mainly print the same list as above, but there might be
     # some more in its list.
-    DEBUG=1 /usr/share/prometheus-node-exporter-collectors/apt_info.py
+    DEBUG=1 /usr/share/prometheus-node-exporter-collectors/apt_info.py >/dev/null
 
 Check the state of each package with `apt policy $package` to determine what
 needs to be done with it. If most cases, the packages can just be purged, but