Metronome will be locked.
Encore locks the move the Pokemon used. Using say Thunder is just the effect of Metronome. The Pokemon doesn't actually know the move Thunder.
The same thing goes for Sleep Talk, since using the other moves is as a result of the effect of Sleep talk.
Nature Power is actually just a standard attack with variational effects, it will be Encored the same as all other attacks.
Source: EXP Points (You can go here too but I'm not sure it'll help)