LED+ is connected directly to battery+, so you you have created a direct short on the cells.
If you are using the leads of the meter, you have to put the leads in as part of the circuit, which of course will alter (lower) the actual reading when the light is assembled again without the meter in circuit.
Using a clamp meter is the way to go - a lot of members have recently got one of the UNI-T UT210E’s