Since you're not using that variable except for in that instance, why not just do:
NS_LOG_INFO("------RECEIVED: " << message.GetSize()
<< " Bitrate = " << (8 * m_totBytes / (Simulator::Now() - m_startTime).GetSeconds())
<< " instBit = " << instBitrate
<< " normBit = " << normBitrate);