SAO PAULO/BRASILIA, Oct 5 (Reuters) - Brazilian assets rallied on Monday after right-wing Senator Flavio Bolsonaro outperformed opinion polls to finish first in the first round of Sunday's presidential election, with the Bovespa (.BVSP), opens new tab benchmark stock index gaining more than 8% and the country's currency strengthening against the US dollar. The eldest son of former President Jair Bolsonaro won 47% of the votes cast and will face leftist incumbent President Luiz Inacio Lula da Silva, who secured about 45% of the vote, in a runoff on October 25. Polls had forecast Lula would lead the first round...