add NIC-level statistics to Packet Capture Statistics dashboard

[mmguero]

(this was split out from #703)

Request for adding NIC-level stats (drops, traffic volume, etc.) to the Packet Capture Statistics dashboard. However, #775 will need to be fixed before we can do this, which is why I split it out from #703. Look at the Resources - Hosts Overview dashboard as we already are using this stuff there and can probably reuse some of it for the Packet Capture Statistics dashboard

• Statistics from the NIC itself (/proc-level)
  • see mmguero-dev/Malcolm@881fc35 for code hooking this up (tl;dr, set a cron expression in ./config/pcap-capture.env's PCAP_IFACE_STATS_CRON_EXPRESSION to determine the stats interval.