I am using a Gems Rotorflow with pulse output fed into a fixed I/O PLC. It gives me a rough idea of flow rate. (The PLC is for my automated startup sequence and monitoring).
These paddlewheel units are also available with setpoint switches installed.
edit: The paddlewheel is also a neat visual indicator...
I don't know how to initialize a software shutdown. But I can automatically hold the power button for 4s, etc. to shut down with the BIOS ...or take more drastic meaures...