Your metric is defined as number such as this =NumberFormat({$_},"#,###.00") and you would want to change the format to currency format (=Currency($_,"#,###.00") to see the desired output "(1,125.6) <----with this in red" . This action I would do if I want this change to be reflected all over Apptio environment.
However, if you are trying to change in a particular report as opposed to global metric then I would suggest creating a new variable in the report. Editing window of the new variable will allow you to define Currency format, as shown below.

Regards,
Prem