I have a Yamaha CP-50 stage piano and I was unable to find a metronome function. Surely metronome is not one of the preset drum settings. Does anyboy know an easy way how to start a metronome on this device?
From the manual it seems like you need to be recording in order to use the metronome. You could try this part, but it's step 4 of the recording process:
Press the [PAGE ►], , or  button to navigate to Pages 3 and 4 of the Record screen, and then turn Knobs 1 to 3 to set the tempo, time signature, and metronome for recording.
If that doesn't work check out the owner's manual and try the whole process — it's on the Support tab of this page.