Whiskey inside a metal flask for a month. Safe for drinking?
Whiskey is quite high in alcohol, on the order of 40% by volume, and is not hospitable to pathogens growing.

The flask is intended to hold liqueur, and so is made from or lined with a food safe material, such as food grade stainless steel (assuming you have one from a reputable manufacturer).

So yes, it should be fine. 

Remember:  when it was distilled, the whiskey was probably held at different stages for long periods in a stainless steel vat.