rtpsource: Don't add non-relevant stats
rtpsource: Don't add non-relevant stats

A lot of the stats are not initialized or valid for any given scenario.

Only including what we need, with ways for the application to know
what to expect, makes the stats much more compact and to-the-point.

pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good