Oxford dictionary: whistle-blower (used especially in newspapers)a person who informs people in authority or the public that the company they work for is doing something wrong or illegal

I am not sure the exact single noun exists. Доносить (сообщить) о нарушениях expresses the idea.

стукач is usually associated with prison contexts (it's prison slang), one who provides information on other prisoners to prison authorities

доноситель is a word popular in the USSR context (one who secretly tells the police about someone doing something or tells their bosses about their colleagues, etc.)

осведомитель - very often it's a police informant

информатор - one who provides information