Heavy metal is often associated with primal roars, dark themes, and apocalyptic instrumentation, but the polished Japanese idol band BABYMETAL keeps only the latter trait and dispenses with the rest, combining a thrashing musical backdrop with bubbly J-pop that could be lifted from a video game. The jarring, if not altogether surreal, contrast—complete with theatrical girly-goth aesthetics and fully choreographed performances—has thrust the group into the international spotlight and continues to convert metal traditionalists and newcomers alike. It’s a phenomenon that feels like it could exist only in the digital era, when notions of genre are a suggestion at best, and the element of surprise continues to pay. In advance of its forthcoming album, “Metal Galaxy,” BABYMETAL performs at Terminal 5, on Sept. 15. It may just be the cutest music you’ve ever headbanged to.