how to measure the streaming bandwidth

The most accurate way to track bandwidth usage over extended periods of time (hours, days, etc) is to parse the access log files generated by the server. When a client disconnects, the total number of bytes transferred to that client is recorded. Sum up all the transfers for individual clients over a fixed period of time, and divide it by the time period, and you get the average bandwidth consumed over that time period.
This method is the most accurate, and is probably what you want to do when accuracy and/or auditing is required (like if you are billing your clients). Unfortunately, it is not "real time". If you want an instantaneous view of bandwidth usage, you can retrieve the byte counts for each application using the Admin APIs.

Hope this helps