The ornamental design for a metronome, as shown and described.
FIG. 1 is a front, right bottom perspective view of a metronome showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The structural elements depicted by broken lines in FIG. 3 have been shown for the purpose of illustrating portions of the metronome that form no part of the claimed design.