Enable mod_status

a2enmod status

Configure mod_status (/etc/apache2/mods-enabled/status.conf on Debian)

<IfModule mod_status.c>
# Allow server status reports generated by mod_status,
# with the URL of http://servername/server-status
# Uncomment and change the ".example.com" to allow
# access from other hosts.
ExtendedStatus On
<Location /server-status>
    SetHandler server-status
    Order deny,allow
    Deny from all
    Allow from

Listen 61709

Restart Apache

/etc/init.d/apache2 restart

Add the following to the end of /etc/zabbix/zabbix_agentd.conf

# Apache Keys
UserParameter=apache.status[*],wget -q -O /dev/null "http://localhost:61709/server-status?auto" && wget -q -O - "http://localhost:61709/server-status?auto" | awk '/$1: / {print $NF}'

Restart the Zabbix agent

/etc/init.d/zabbix-agent restart