`Filecoin.NetBandwidthStats` return real metrics

# Summary

Right now we only have a stub for `Filecoin.NetBandwidthStats` that returns all zeroes. The stub needs to be there, though, otherwise some widget fail completely (even if they don't use the data...).

There are no ready interfaces for this through libp2p AFAIK so this might need a bigger change. Maybe there's already something in `rust-libp2p`, though, not sure. I recall one can enable all p2p metrics somehow, but that might be an overkill. To be measured.

# Completion Criteria

- [ ] add real metrics to `NetBandwidthStats` - measure they really reflect reality.

# Additional Links & Resources

Start at the Filecoin.NetBandwidthStats entry point and investigate whether libp2p or rust-libp2p exposes usable peer bandwidth metrics. Confirm that the selected measurements reflect reality, while preserving the existing stub behavior needed by widgets.

Written by the indexing model from the issue text.