Add metrics collection support to nftables set-only mode

[jarppiko]

Summary

This PR solves #291. It stops crowdsec-firewall-bouncer flooding crowdsec-firewall-bouncer.log by adding basic support for metrics collection to set-only mode.

As explained in #291 , it is not a perfect solution since it allows metrics collection only from one chain.

Description of changes

File changed: pkg/nftables/metrics.go

1. Change collectDroppedPackets() to take chain name directly
2. Move string concatenation (chain-hook) from it collectDroppedPackets() to collectDropped()
3. Add if c.setOnly clause to collectDropped() to handle both the cases

Further considerations

Collecting metrics from all chains / hooks in set-only mode would require a way to tell the bouncer which chains are being used. This could be done with the following changes (not included in the PR)

1. Change nftables_hooks config option to contain actual chain names and the remove the use of undocumented string concatenation to create nftables chain names from chain (basename) and hooks
2. Use the same nftables_hooks config option in set-only mode to tell the bouncer from which chains to collect metrics from

[LaurenceJJones]

lgtm tested