The ornamental design for a straingauge, as shown and described.
FIG. 1 is a top plan view of a strain gauge, showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof; and,
FIG. 4 is a bottom plan view thereof.
The broken lines and the uneven-length broken lines shown in the drawings represent portions of the strain gauge that form no part of the claimed design. All sides not shown form no part of the claimed design.