Key logger/watcher suggestion
I would assume the repair technician has total control over your files since he or she controls physical access, can remove the storage and image it, knows how to reset passwords, make secondary admin accounts, etc.

Most technicians have high standards and should explain the procedures they will take to guard your data, but you'll still be extending trust to someone with the training and opportunity to make a copy of your data and/or poke around.

As to observing them, unless you reported a software problem and they are going to fix your software, you won't learn anything by logging the keys pressed.

They will use their computers to do research and likely boot off their software to see if your problem is your settings or your particular copy and version of the OS.

If you install audio or video recording software, you may want to get legal advice since you don't want to break any laws spying on them. Again, check the terms of service - you will likely be granting them permission to erase, modify and inspect your computer when you hand them the hardware.

If your goal is to track what they do, why not ask them to provide you with steps and then check their work by running tmutil compare to see which files changed after service from a backup that happened before service?

I've never found key loggers to be useful and other recording software is buggy and slow IMO.

What I do is delete the drive entirely and give them a test account or use FileVault to encrypt everything you care to "watch" and just not give them the password or key to decrypt the volume if for some reason you din't want unauthorized access.