The ornamental design for a coffee tamper, as shown and described.
Fig. 1.1 is a front view of a coffee tamper; the rear, left and right side views are omitted because those views are identical to fig. 1.1; fig. 1.2 is a top view of the coffee tamper of fig. 1.1; and fig. 1.3 is a bottom view of the coffee tamper of fig. 1.1.
In the drawings, the broken lines show unclaimed subject matter only and form no part of the claimed design.