I want my circuit to be isolated but I can't connect primary of transformer to grid because I can't afford 10Amps going through.

 

This module is based around the ZMPT101B precision voltage transformer and includes support circuitry. There is a lot of documentation on using it.

​

 

The ZMPT107-1 may be useful: https://5nrorwxhmqqijik.leadongcdn.com/ZMPT107-1+specification-aidiqBqoKomRilSqqnnkikq.pdf

 

I can't connect primary of transformer to grid because I can't afford 10Amps going through.

A transformer pulls only a small magnetizing current (a few percent of the maximum load current) when it has no secondary load.

 

Yes, this is done all the time. This is especially true in measuring medium and high voltages (power line AC voltages). There are specialized transformers for these markets called Potential Transformers that keeps the voltage drop error low, due to controlling leakage inductance and core magnetizing current.

I have used this technique myself many times in low voltage designs. Your milage may vary depending upon what accuracy and response time you need. You can also use calibration ideas to correct measurement errors.

 

I want my circuit to be isolated but I can't connect primary of transformer to grid because I can't afford 10Amps going through.

There is no reason for your voltage measurement to draw 10 amps, even if that is the rating of your power source. For example, if I measure 240 VAC with my digital voltmeter across a 30 amp source my meter does not dissipate 7.2 kilowatts.

The power dissipated by a load is determined mainly (no pun) by the load.

 

You can get the ZMPT101B from https://www.micro-transformer.com/ and a whole lot of other handy measurement transformers for voltage or current.
The ZMPT101B is a 1:1 current transformer which works at 2mA max. If you drive it via a 230k resistor and put a 1k resistor load on the output you will get 1V rms which you can measure with your microcontroller.