How to measure thrust produced from combustion chamber of Gas Turbine Engine?
Option 1: Thrust is a force, and Force = mass * acceleration, and acceleration is change in velocity.  So you need to measure the mass flow, the velocity upstream and the velocity downstream.  It is probably sufficient to just measure the air mass flow upstream, and measure the fuel flow, and not measure the air mass flow downstream.

Option 2: instead of measuring the air, measure the reaction force.  i.e. at some point, you have to support your combustion chamber.  It will be bolted down to the floor somewhere.  Where ever that is, get a load cell. That is a thing that directly measures force.

But again, with either of these two options, I would expect that your measurement thrust measurement will be pretty close to zero.  A combustion chamber will not generate a large thrust on its own.  You need a nozzle in order to generate a significant amount of thrust.