[mac] count out-of-band Tx frames (#4936)

include/openthread/link.h

@@ -191,7 +191,7 @@ typedef struct otMacCounters
     /**
      * The total number of unique other MAC frame transmission requests.
      *
-     * This counter is currently unused.
+     * This counter is currently used for counting out-of-band frames.
      *
      */
     uint32_t mTxOther;

src/core/mac/mac.cpp

@@ -1369,6 +1369,8 @@ void Mac::HandleTransmitDone(TxFrame &aFrame, RxFrame *aAckFrame, otError aError
 #endif
 
     case kOperationTransmitOutOfBandFrame:
+        // count Oob frames
+        mCounters.mTxOther++;
         FinishOperation();
         PerformNextOperation();
         break;