[Packetbeat] Report AF_PACKET statistics in metrics

[taylor-swanson]

The afpacket library exposes a variety of statistics on the AF_PACKET socket and buffer. Among them are:

• Socket packets (packets delivered to the shared buffer by the kernel)
• Socket drops (packets dropped by the kernel, i.e. buffer full)
• Socket queue freezes
• Buffer packets (packets read from buffer by packetbeat)
• Buffer polls (number of blocking syscalls made waiting for packets)

"Buffer" might be the wrong word to use for the metric name.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)