I have a 2014 ML350 Bluetec. I just got a call that mass airflow sensor needs to be replaced. Can anybody please let me know if this will be covered by the federal emission warranty ?

no...MAF has nothing to do with emissions system

For diesel engines, the MAF output is used to control the EGR, one of the emission systems. However, I doubt it will be covered by warranty--perhaps after the emissions recall is performed that also expands the engine warranty coverage.