Cronista translates to "chronicler" or "historian". It's an interesting and somewhat formal word to find in a rap song, making it stand out.
Duki uses it metaphorically to say, "De las pena' y tristeza' que tuve en mi vida te hiciste cronista" (Of the pains and sadness I had in my life, you became the chronicler). He's implying that this person was so intimately involved in his life that they documented or witnessed all his struggles, highlighting a deep and significant past relationship.
Duki looks back on the early days with someone he loves, before fame and touring pulled them apart. He remembers late nights, fast drives, and being young together — a time before schedules, cities, and distance got in the way.
Now he is trying to win her back before it is too late, admitting that loyalty and love do not always survive success. The chorus, "antes de perderte" ("before losing you"), is his promise to fix things while there is still a chance.