Concern about metadata in rosetta

[lleon95]

I am a bit concerned about the use of the GstInference methods. If you check other elements, you may check that GstInference provide functions for printing, building the metadata blocks, and so on. Please, take them as a reference:

• https://github.com/RidgeRun/gst-inference/blob/master/ext/r2inference/gstinceptionv2.c#L238
• https://github.com/RidgeRun/gst-inference/blob/master/ext/r2inference/gsttinyyolov3.c#L289

The prediction class is declared here: https://github.com/RidgeRun/gst-inference/blob/a1428f86f967a7e21ea715a4d055d3bdffe3eb9b/gst-libs/gst/r2inference/gstinferenceprediction.h

and defined here: https://github.com/RidgeRun/gst-inference/blob/55cad49398398f1fcc424f76244e65585c7cd8f4/gst-libs/gst/r2inference/gstinferenceprediction.c

Originally posted by @lleon95 in https://github.com/RidgeRun/gst-inference/pull/340#r754371900