BOGOTA, Colombia — Colombians hit the polls in the first round of their presidential election Sunday to choose who they want to guide Latin America’s third most populous nation. The country is going through a troubled moment marked by soaring cocaine production, a shaky peace accord with a Marxist rebel group and an influx of Venezuelan migrants flooding over the border to escape an economic crisis next door. The conservative 41-year-old lawyer Iván Duque was leading in pre-election polls, followed by the progressive 58-year-old economist and ex-mayor of Bogotá, Gustavo Petro. Other candidates are a mathematician who was the centrist mayor of Medellín, Sergio Fajardo, and a former vice-president, Germán Vargas Lleras, on the center-right. No candidate was expected to win outright Sunday, and there will likely be a run-off, probably involving Duque and Petro.