Cherry-pick #14839 to 7.x: [Metricbeat] Add STAN (NATS streaming) module

[ChrsMark]

Cherry-pick of PR #14839 to 7.x branch. Original message:

This PR cherry-picks the commits of @devon-kim
from 7.2...devon-kim:ops-1844-stan-metricbeat so as to be merged to beats/master as part of #14629.

cc: @exekias

Manual testing

1. Build the Docker image: docker build -t stan_test . (run this command under x-pack/metricbeat/module/stan/_meta)
2. Start STAN server: docker run -p 8222:8222 stan_test
3. Enable the module and run Metricbeat.
   (Note that default fetching period is 60secs, so this might should be changed for the testing)

[mtojek]

As long as it's only 7.x backport, feel free to merge it :)

[mtojek]

I'm afraid you will need to run also mage fmt update (see Travis output).

[devon-kim]

Travis error is from time-outs now in heartbeat and the metricbeat test suites due to the test stdout being swallowed. Simple fix for the 7.x doc change otherwise.

[ChrsMark]

I restarted the failed jobs. We need to solve the conflict in Changelog. I propose we remove completely the docs link to avoid these issues. Wdyt?

[ChrsMark]

I restarted the failed jobs. We need to solve the conflict in Changelog. I propose we remove completely the docs link to avoid these issues. Wdyt?

What I said about removing docs is not correct. These files are auto generated.