Munin falsely reports high MySQL InnoDB free tablespace usage

Symptoms

Munin reports high MySQL InnoDB free tablespace usage even though it is set to autoextend.

Description 

Munin reports high MySQL InnoDB free tablespace usage even though it is set to autoextend, for example:

# mysql -e "SHOW VARIABLES LIKE 'innodb_data_file_path'"
+-----------------------+------------------------+
| Variable_name         | Value                  |
+-----------------------+------------------------+
| innodb_data_file_path | ibdata1:12M:autoextend |
+-----------------------+------------------------+

Workaround

Create file /etc/munin/plugin-conf.d/mysql_innodb with the contents:

[mysql_innodb]
env.warning 0
env.critical 0

Then restart  munin

systemctl restart munin-node