Because of the fact that Munin by default throws a warning at 2GB free tablespace and a critical alert at 1GB free tablespace, you will probably get a permanent Munin warning for MySQL InnoDB free tablespace. MySQL will by default create a 10MB autoexpanding tablespace, which autoextends as needed, thus this warning should be disregarded.
A simple fix is to disable the alerts. For this, create the following file:
paste the following code:
[mysql_innodb] env.warning 0 env.critical 0
and restart munin-node: