Hey, welcome Francisco!
I’ll also answer Best way to get bandwitdth in suricata? as these two almost seem like a duplicate.
I think you are on the right track - decoder.bytes can be used to calculate the bandwidth that your Suricata instance is actually able to process.
To calculate bandwidth that is currently coming to your NIC directly - for that I would probably use ethtool stats. Unfortunately, I am not aware that Suricata could calculate directly in some stats output.
I feel like generally, users observe these stats when Suricata is connected to an SIEM of some kind - e.g. ELK. To have it directly in Suricata stats I guess it could be a nice beginner-friendly contribution to the stats.