Enable Guest/Instance network interface(s) metrics

[zeeshanlakhani]

• Omicron PR with new instance-network-interface.toml schema and addition of interface_id/nic_id to NetworkInterfaceRequest => [network metrics] instance network interface schema omicron#6414
• Have viona driver expose kstats directly (related: https://code.illumos.org/c/illumos-gate/+/3630, cc @pfmooney )
• Plumb interface_id/nic_id through to VirtioNic Device => [network metrics] instance network interface metrics #747
• Add Kstat sampler target for network interface instance stats (in addition to current sampler) => [network metrics] instance network interface metrics #747