Hello there and welcome to the Wowza support forum.
Please take a look at this module and see if it suits your needs:
Module to calculate bitrate of a video on demand stream
For live or static streams, the Quality of Service metrics in NetStream can show what you are actually consuming client-side:
import flash.net.NetStreamInfo;
// do this on interval or timer:
var nsi:NetStreamInfo;
nsi = netstream.info;
trace(nsi.dataBytesPerSecond);
For live streams, server-side, you can use IPerformanceCounter. There is getMessagesInCountRate() and getMessagesOutCountRate() which should give a snapshot.
Kind regards,
Salvadore