Hi,
Depending on the installation, either in syslog or in a file. IIRC the default in Debian packages uses the syslog.
One immediate thing to test - set the IdleTimeout to 5 minutes and see if that changes something.
The docker+machine driver is known to be troublesome and also not developed anymore to my knowledge. In combination with OpenStack, this is likely your best bet I’d say. A friend of mine has been implementing a similar setup at NETWAYS, where I had blogged about it.
You might have read that GitLab 12.10 now supports AWS autoscaling natively within Fargate. Previously this setup also used docker+machine which has now been replaced with a custom driver using the custom executor.
I’m not sure if that’s possible with OpenStack too, or if plans go there. Since I see OVH in your config, it may be an idea to ask them too, maybe they have some experience in this region 
Peeking into the LXD driver example, this invokes some scripts to run the driver stages. That may be the first attempt to look into OpenStack then. For implementing a real driver in Golang, you probably need to mimic the AWS Fargate logic - this uses the AWS SDK for Golang. I’d love to dive deeper but too many open tasks
Cheers,
Michael