I am assuming you are referring to roasted coffee beans infused with flavor. If so, you can consider that to be equivalent to plain black coffee in terms of calorie and sugar content.
Coffee beans are flavored with chemical solvents, not sugar. A hazelnut roast does not have more sugar than a non-flavored one.
Mind you, this is very different from drinking regular coffee with added flavor shots.
At Starbucks, a tall coffee with a syrup shot has 8 grams (2 teaspoons) of added sugar, a grande clocks in at 12 added grams (3 teaspoons), and a flavored Venti contains an additional 20 grams (5 teaspoons) of sugar.
Powdered flavored coffees – to which you simply add hot water – also contain sugar. For example, a tablespoon (one serving) of International Foods’ French Vanilla coffee powder contains 8 grams (2 teaspoons) of sugar.