Bridge rectifier & transformer to measure mains
A transformer is required for safety (to isolate from the high voltage input).  A common doorbell transformer can be directly connected to AC; if yours is NOT a doorbell transformer, it may lack internal protection (fuse link, resistance) against short circuits, and an external fuse and maybe a limit resistor are recommended.







simulate this circuit &ndash; Schematic created using CircuitLab

The Arduino analog inputs have a permissible range of 0 to +3.3V when using the builtin 3.3V regulator as analog reference, so R2 and R3 sets DC level, and R1 and the S.A.T. (select according to test) resistor will attenuate the input voltage. I'd aim for 2V peak-to-peak, indicated by AC voltage on the S.A.T. resistor of 0.7 VAC.   Don't trust the transformer ratio (because doorbell transformers, while functional, are not always accurate).

There's no need for diodes, the AC is easier to read without adding complications.