As others have said, the metal part isn't cooler - it's an artifact of the thermal camera.

It comes down to emissivity, and reflections.

A "thermal camera" or "(non-contact) IR thermometer" measures the radiated heat (long-wavelength thermal energy) emitted by the object you're pointing it at.

All objects emit the same spectrum (strictly spectral distribution) of radiation, depending only on their temperature - look up Blackbody radiator and colour-temperature.

The absolute amount of energy ("brightness") also varies strongly with temperature, but depends also on a property of the material, known as its emissivity.

For simplicity, IR cameras typically only measure the strength of emission at one wavelength (usually somewhere around 3-12µm), and determine the temperature by the "brightness" at that wavelength.

For most common matt/dielectric (non-metal) materials, the emissivity is 97-99% - which is the default calibration for an IR thermometer.... but metals, especially polished shiny ones, and especially gold has a lower emissivity, so the IR thermometer will under-read.

You can look up the emissivity for different materials and set the IR thermometer calibration accordingly, to get correct readings ... but be aware that metals can also look like mirrors, and you may "read" the temperature of the thing in the reflection in the metal, rather than the metal itself - or somewhere between the two.

If you wear a metal ring (especially a gold one) on your finger an point the thermal camera at your hand, you'll see the ring is darker (and reads "colder") - even though its true temperature is likely to be close to that of your fingers.

This is something that not enough people know about IR thermometers and IR cameras.

It's a physics thing! :-)