feature request: metricbeat: support for ntp

Enhance metricbeat to allow status monitoring of time synchronisation daemons.

Time drift is a big thing(TM) in financial services and other systems. It would be a great feature of metricbeat to be able to pull in various time parameters and store them historically.

This is a clone of: https://github.com/elastic/beats/issues/2926 which was closed before any work was carried out.

PTP, chrony, or NTP are widely used to synchronise system time with a network clock. Offset from the external clock is calculated, then the clock is gently (sometimes) drifted towards the external clock signal rather than jumping right to it. The offset is therefore something which is of great use if it can be stored historically.

This helps show whether transactions actually happened at the expected time, and would allow systems to locally monitor their own compliance with various financial regulations.

The issue names Metricbeat and the prior issue #2926 but no files, tests, or entry points. Start by reviewing Metricbeat's existing module structure and issue #2926, then clarify whether support must cover PTP, chrony, NTP, or all three and which time parameters are required; done means status and offset data can be collected for historical monitoring.

Written by the indexing model from the issue text.