210.12(A) ex1 covers that

Quote
Exception No. 1: If RMC, IMC, EMT, Type MC, or steel
armored Type AC cables meeting the requirements of
250.118 and metal outlet and junction boxes are installed
for the portion of the branch circuit between the branchcircuit
overcurrent device and the first outlet, it shall be
permitted to install an outlet branch-circuit type AFCI at
the first outlet to provide protection for the remaining portion
of the branch circuit.

Greg Fretwell