prometheus.promtool cannot write debug file

The home plug was added at some point after this bug was raised, but we're now seeing a different issue:

jsimpso@kif:~$ promtool debug metrics http://127.0.0.1:9103
collecting: http://127.0.0.1:9103/metrics
Compiling debug information complete, all files written in "debug.tar.gz".

jsimpso@kif:~$ promtool debug metrics http://localhost:9103
collecting: http://localhost:9103/metrics
error completing debug command: error executing HTTP request: Get "http://localhost:9103/metrics": dial tcp: lookup localhost on [::1]:53: read udp [::1]:48376->[::1]:53: read: connection refused

= AppArmor =
Time: Aug 23 11:46:41
Log: apparmor="DENIED" operation="open" profile="snap.prometheus.promtool" name="/run/systemd/resolve/stub-resolv.conf" pid=2183338 comm="promtool" requested_mask="r" denied_mask="r" fsuid=1000 ouid=101
File: /run/systemd/resolve/stub-resolv.conf (read)
Suggestions:
* adjust program to use $SNAP_DATA
* adjust program to use /run/shm/snap.$SNAP_NAME.*
* adjust program to use /run/snap.$SNAP_NAME.*
* adjust snap to use snap layouts (https://forum.snapcraft.io/t/snap-layouts/7207)
* add one of 'firewall-control, fwupd, network, network-bind, network-control, network-observe' to 'plugs'

= AppArmor =
Time: Aug 23 11:46:41
Log: apparmor="DENIED" operation="open" profile="snap.prometheus.promtool" name="/etc/hosts" pid=2183338 comm="promtool" requested_mask="r" denied_mask="r" fsuid=1000 ouid=0
File: /etc/hosts (read)
Suggestions:
* adjust program to read necessary files from $SNAP, $SNAP_DATA, $SNAP_COMMON, $SNAP_USER_DATA or $SNAP_USER_COMMON
* adjust snap to use snap layouts (https://forum.snapcraft.io/t/snap-layouts/7207)
* add 'system-files (see https://forum.snapcraft.io/t/the-system-files-interface for acceptance criteria)' to 'plugs'