Logging sub-libp2p doesn't work anymore

[tomaka]

Doing polkadot --log sub-libp2p doesn't show any of the sub-libp2p logs anymore.

Tried with Substrate master (commit f6c305a)

cc @andresilva @sorpaas

[bkchr]

This is an upstream "bug": tokio-rs/tracing#1012

[tomaka]

Considering that it's the second time that it breaks, I wonder if we should not admit defeat and rename the logging target to sub_libp2p.

[bkchr]

I think it is not the only logging target with a dash.

After upstream has released a new version, we should introduce a test to make sure we catch this stuff.

And I also hope that we don't again switch the logging implementation again in the near future.