gst-tracers: Add bitrate tracer

The bitrate tracer will measure the bits per second that flow through the different src pads in all the elements in the pipeline.

Test it by running

GST_TRACERS=bitrate GST_DEBUG=GST_TRACER:7 gst-launch-1.0 videorate ! fakesink

The output should be something similar to:

0:00:01.557178000 99281    0x14afc9400 TRACE             GST_TRACER :0:: bitrate, element=(string)videotestsrc0, src-pad=(string)src, bitrate=(guint64)98304000;