Safety isn't a problem. Remember that absolute voltage alone is not dangerous, it is potential difference between two points that creates the safety issues. In line energy meters, the whole monitoring circuit is at (or close to) line voltage so as long as nothing at a different potential comes in contact there is no risk.
As Klaus points out, you need to be able to read the result somehow, and that requires interaction outside the circuit. For meters with digital readout it can obviously be read through a transparent window, for automated readings it is done using short range wireless, usually Bluetooth, Zigbee or something similar.
Brian.