Hi,
Some important questions:
* are you experienced to work with dangerous high voltage?
* is it a hobby project or a professional for mass production?
* do you need isolation?
***
The dangerous, but simple and most precise solution is a resistive divider, adding offset, ADC. Choose a sampling frequency of integer multiple of mains frequency (at least 16), and do all the RMS math inside.
The safe way is with a transformer. If you need phase information to a current, then you will see, that especially low wattage transformers introduce voltage phase shift from input to output.
Klaus