Metralla literally translates to "shrapnel", the metal fragments from an exploding bomb. It's a very intense and unusual word to find in a romantic song!
Juanes uses it in the powerful metaphor, "Me siento indestructible como un cañón de metralla" (I feel indestructible like a shrapnel cannon). He uses this extreme, military-grade imagery to express just how incredibly strong and invincible his partner's love makes him feel. This unexpected word choice makes the feeling he's describing truly unforgettable.
This song by Colombian superstar Juanes is a massive declaration of how love can make every single bad feeling just disappear. He's not just happy; he's completely transformed. His partner's words have almost given him superpowers. When they say "que me amas" [that you love me], he feels "sublime como el tibio sol de la mañana" [sublime like the warm morning sun]. When they ask him "que te ame" [to love you], he feels "invencible como el huracán más temible" [invincible like the most fearsome hurricane]. It's an intense and sweet way to describe the power of affection.
But the real heart of the song is the chorus, where he lists everything he doesn't feel anymore because of this love. He says, "No siento penas ni dolores de cabeza / Ni confusión de ninguna naturaleza" [I don't feel sorrows or headaches / Or confusion of any kind]. All the normal, everyday worries are just gone. What's left? Only one thing: "Solo siento amor" [I only feel love]. It's a simple, direct, and powerful message about love's ability to clear away all the noise and leave only the good stuff behind.