pipeline: inputs: docker-metrics: add container path config

[hkhelif]

What type of PR is this?

documentation

What does this PR do / Why do we need it?:

At times the user might need to have their docker Daemon data directory setup somewhere else other than the default path. Currently the Docker Log Based Metrics plugin does not pick it up automatically and has it hard coded here.

We can simply pass path.containers to the config and that will do the trick as the plugin code is also looking at that same config here

This can significantly save the end user research time and also opens their eyes to the capability of the docker metrics plugin

Testing done on this change:

This was tested, let's assume my docker data-root is set to /some-path/docker the bellow config worked, and the plugin was able to access the containers as expected using this config

[INPUT]
    Name                              docker
    tag                               docker.containers
    path.containers                   /some-path/docker/containers

I have also tried adding DEFAULT_CONTAINERS_PATH to /etc/sysconfig/fluent-bit as per this document, but that did not work.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[esmerel]

This looks fine to me from a writing perspective, but should get a @fluent/fluent-bit-maintainers approval on the technical aspect.