gst-tracers: Add bufferrate tracer

This feature adds the bufferrate tracer to the set of GStreamer core tracers. This tracer logs the number of buffers and bits per second for every src pad in the pipeline.

To test it you can run:

GST_DEBUG=GST_TRACER:7 GST_TRACERS="bufferrate" gst-launch-1.0 videotestsrc is-live=true ! queue ! fakesink

And you should see an output similar to this:

0:00:06.627506545 34694 0x55de22c2fd90 TRACE             GST_TRACER :0:: bufferrate, element=(string)videotestsrc0, src-pad=(string)src, buffers-per-second=(double)30,000000, bits-per-second=(double)147456000,000000;
0:00:06.627573094 34694 0x55de22c2fd90 TRACE             GST_TRACER :0:: bufferrate, element=(string)queue0, src-pad=(string)src, buffers-per-second=(double)30,000000, bits-per-second=(double)147456000,000000;