The Design consists of the features of shape, ornamentation and configuration, of the LOW-PROFILE FITNESS TRACKER shown in the drawings. Portions shown in dashed lines form no part of the design. Drawings of the Design are included, in which: Figure 1 is an isometric view of the first variant of the low-profile fitness tracker. Figure 2 is a front view of the first variant of the low-profile fitness tracker. Figure 3 is a back view of the first variant of the low-profile fitness tracker. Figure 4 is a top view of the first variant of the low-profile fitness tracker. Figure 5 is a bottom view of the first variant of the low-profile fitness tracker. Figure 6 is a side view of the first variant of the low-profile fitness tracker. Figure 7 is an opposite side view of the first variant of the low-profile fitness tracker. Figure 8 is a top oblique view of the first variant of the low-profile fitness tracker. Figure 9 is a first bottom oblique view of the first variant of the low-profile fitness tracker. Figure 10 is a second bottom oblique view of the first variant of the low-profile fitness tracker. Figure 11 is an isometric view of the first variant of the low-profile fitness tracker in a band-closed configuration, such as is formed when the ends of the band are fastened together as shown. Figure 12 is a bottom oblique view of the first variant of the low-profile fitness tracker of Figure 11 in the band-closed configuration. Figure 13 is an exploded view of the first variant of the low-profile fitness tracker. Figure 14 is an isometric view of the second variant of the low-profile fitness tracker. Figure 15 is a front view of the second variant of the low-profile fitness tracker. Figure 16 is a back view of the second variant of the low-profile fitness tracker. Figure 17 is a top view of the second variant of the low-profile fitness tracker. Figure 18 is a bottom view of the second variant of the low-profile fitness tracker. Figure 19 is a side view of the second variant of the low-profile fitness tracker. Figure 20 is an opposite side view of the second variant of the low-profile fitness tracker of. Figure 21 is a top oblique view of the second variant of the low-profile fitness tracker. Figure 22 is a first bottom oblique view of the second variant of the low-profile fitness tracker. Figure 23 is a second bottom oblique view of the second variant of the low-profile fitness tracker. Figure 24 is an isometric view of the second variant of the low-profile fitness tracker in a band-closed configuration, such as is formed when the ends of the band are fastened together as shown. Figure 25 is a bottom oblique view of the second variant of the low-profile fitness tracker of Figure 24 in the band-closed configuration. Figure 26 is an exploded view of the second variant of the low-profile fitness tracker.