/usr/share/munin/plugins/hddtemp is in munin-plugins-extra 2.0.19-3ubuntu0.3.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37  | #!/bin/sh
# -*- sh -*-
#
# Plugin to monitor harddrive temperatures through SMART.
#
# This plugin is an alternative to hddtemp_smartctl, which is
# the preferred one.
#
# client-conf.d/-options:
# 	
# 	drives		-- List drives to monitor. E.g. "hda hdc".
#
#%# family=contrib
HDDTEMP=/usr/sbin/hddtemp
if [ "$1" = "autoconf" ]; then
        if [ -x "$HDDTEMP" ]; then
                echo yes
                exit 0
        else
                echo no
                exit 0
        fi
fi
if [ "$1" = "config" ]; then
        echo 'graph_title HDD temperature'
        echo 'graph_args --base 1000 -l 0'
        echo 'graph_vlabel Degrees Celsius'
        echo 'graph_category sensors'
        for a in $drives ; do echo $a.label $a ; done
        exit 0
fi
for a in $drives ; do printf "$a.value $(hddtemp -n -q /dev/$a)\n" ; done
 |