I have used the Ranco ETC-111000. It is an electronic control unit and the sensor cable is 8 ft, but you can add many ft of cable to it because the additional resistance doesn't affect the measurement. You can route the sensor and power wires out the back so you don't have ugly wires exposed on the wall.
If the boiler is using a 24 Volt system you can get the ETC-112000 model from Grainger and use low voltage power for the electronics.
I would put a tube in the concrete so the probe could be put in after pouring and removed for repair or replacement.
If you have access to the bottom of the floor you can curve the tube a bit so the open end is exposed and the other end is embedded where you want it in the concrete. The sensor is 1/4" diameter and about 2" song so a 3/8" ID tube will be big enough.
For heat-transfer reasons you will get the best accuracy if you put the tube in so that maybe 2 ft of tube is embedded in the concrete and located approximately equidistant from any pair of heating elements (wires or tubes).
If you search on the famous auction site for RANCO ETC-111000 Microprocessor Temperature Control you will find some for sale.