How to measure the current of a high voltage CO2 Laser?
When High Voltage is involved, safe is a matter of perspective. I would consider it safe to place a shunt resistor or other current measurement system system on the low side of the laser tube as long as no humans are in contact with the device and it is isolated from everything else. For example, an old analog amp meter gauge that is plastic and mounted inside of an enclosure securely, I would consider safe because it is not very conductive and can be easily viewed without the need to hold it.

In a commercial solution, there would be an electrical isolation set up. Some sensor on the high voltage line, then a physical/electrical barrier that the signal is routed across via light or magnetics (a non conductive medium rated at-least 1.5x the working voltage of the laser most likely) to a device that is low voltage and safe to handle or interface to other instrumentation.