packetbeat: add metrics for TCP header flags

[efd6]

TCP flags are potentially useful for early-detecting/diagnosing network outages. This is not a feature that packetbeat currently provides, but would be useful and has been requested.

Logging every TCP packet's header as an event in packetbeat would be onerously expensive. However, it would be reasonable to add a set of metrics for the TCP header flags. This would allow detection of significant changes in e.g. RST flag observation in order to make early responses to network degradation.

[elasticmachine]

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