Merge the Elastic Logging Plugin to master

[fearful-symmetry]

See #13990 and #13761

What is this?

This is a new beat that takes the form of a docker plugin for logs. The only release artifact takes the form of a plugin (which is basically a runc container) which lives in a docker repo. Aside from the different release artifacts, this operates under the hood as a normal beat. It uses libbeat, and takes many of the same configuration options as any other beat.

How to test

Once you have the branch checked out, navigate to x-pack/dockerlogbeat. Run mage package on a host with docker to generate the plugin.

You can test like this:

docker run --log-driver=elastic/elastic-logging-driver:8.0.0 \
--log-opt output.elasticsearch.hosts="172.18.0.2:9200" \
--log-opt output.elasticsearch.index="dockerbeat-test" \ 
-it debian:jessie /bin/bash`

I've never merged a new beat to master, so I'm not entirely sure what needs to happen.

[urso]

@fearful-symmetry @dedemorton Do we have any docs planned for this project?

[fearful-symmetry]

@urso @dedemorton Docs are on my list on #13990 honestly have No idea where to start with proper asciidocs.

[dedemorton]

I told Alex that I would help them set up some skeleton asciidoc files when I get back from vacation in January. I'm planning some structural changes that I want to make in parallel for some stuff that I've been stalled on for awhile.

Forgot to add...for now, I think it's OK for Alex to capture some basic docs in a readme file.

[urso]

@dedemorton @fearful-symmetry Thanks for the clarification. I guess we can proceed merging the plugin then.