What is this flat metallic component?

What is the metallic component, labeled Y1, just at the right of the integrated circuit and at the left of the small C4 and C5 capacitors?

It is a 24 MHz crystal (hence the "24.000" marking on the top).

The component designator letter "Y" is commonly used for crystals.

Capacitors C4 and C5 will be the load capacitors for the crystal.

It is a clock crystal as part of the MCU clock circuit . That is also common for reference designators "Y"

24.000 indicates frequency 20pf indicates loading

C4 and C5 are capacitors used as part of the oscillator circuit.