I am a dual U.S./Serbian citizen living in Serbia and currently on maternity leave, fully paid by the Serbian government. I cannot find any information on whether I’m supposed to report that as a foreign income when filing my taxes for 2017?
As a US citizen, you will need to report this income on line 21 of your 1040 (Other income).
As this maternity benefit is paid by the Serbian government and not your employer, it doesn't appear to qualify as foreign earned income, which the IRS describes as "wages, salaries, professional fees, and other compensation received for personal services you performed in a foreign country".
If you paid income tax in Serbia on these benefits, you might be able to claim foreign tax credit to be offset against your US income tax liability. You would list both the income and the tax paid on Form 1116.