Improve observability of coil egress's proxying traffic

[terassyi]

What

Now coil egress doesn't give enough information about its proxying traffic.
Therefore, in this issue, I plane to add some metrics for egress pods traffic and connection tracking and add useful commands in the coil image.

How

metrics

I want to add following metrics.

• coil_egress_client_pod
• coil_egress_nf_conntrack_entries_limit
• coil_egress_nf_conntrack_entries
• coil_egress_masqueraded_packets_total
• coil_egress_masqueraded_bytes_total

Please see this for details.

• https://github.com/cybozu-go/coil/pull/306/files#diff-103fdde19805bbb61134b9c62432a3e64c8b7c66f7b0197110c96b9f6b2c0313

command

I want to add conntrack command.

https://conntrack-tools.netfilter.org/

Checklist

• Finish implentation of the issue
• Test all functions
• Have enough logs to trace activities
• Notify developers of necessary actions

Tasks

Preview Give feedback

1. Add conntrack command into the coil image #305
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2. Add egress related metrics #306
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