BOSTON — About 5,000 uniformed and undercover police officers, surveillance drones, bomb- and chemical-sniffing dogs and heavy trucks blocking streets are just some of the security measures being used to protect this year's Boston Marathon, law enforcement officials said Tuesday.Authorities are taking no risks when it comes to the 122nd running of the marathon, even though there are no known threats to the April 16 race that goes through eight cities and towns.