We claim the ornamental design for a metal tongue drum, as shown and described.
FIG. 1 is a top plan view of a metal tongue drum musical instrument showing my new design; and,
FIG. 2 is a front perspective view thereof.
The broken lines in the figures illustrate portions of the metal tongue drum and form no part of the claimed design. The broken lines surrounding the incomplete circular shape in the drawings depict the boundary of the claim and form no part of the claim.