Non-isolated mains circuit and Ethernet
While technically Ethernet provides (barely) enough isolation, I would not rely on it to provide for your safety.  You also have other issues to consider.  

One issue is that the Ethernet transformer has a parasitic capacitor between the primary and secondary.  If your MCU circuit jumps up 100-ish volts when you connect it to the AC line, a pulse will be sent out the Ethernet cable.  Many devices have protection on their ports to prevent damage, but many devices don't.  I have blown up many devices in this way.

Then there is your personal safety while debugging this type of circuit.  One tiny slipped probe while debugging could make for a very bad day.  

Of course your real problem is in doing current sensing without requiring expensive isolation transformers.  I suggest that you look for hall effect current sensors.  Here's a link to some.