Add custom tail kubernetes tag example

[VincentSchmid]

Adding an example of how to get the kube.<namespace_name>.<pod_name>.<container_name>.<docker_id> tag in the Tail Input to actually work.

Also, fixing the regex in the Tail documentation to reflect how it worked for me.

Now that I begin to understand what is actually happening, I realize you don't need the above notation to split by namespace, pod, container, or Docker ID. However, for me, the above tag feels more intuitive. From my online research, many people have struggled with this, and there are blog posts that arguably do it worse (no disrespect, these two articles are still helpful):

https://blog.omerh.me/post/2020/05/splitting-kubernetes-logs-by-namespaces-with-fluentbit/

This could work (though it did not for me):
https://www.agilicus.com/routing-to-multiple-outputs-by-kubernetes-namespace-in-fluent-bit/

[lecaros]

Hi @VincentSchmid, thanks for this.
Please review our contributing guidelines and modify your PR accordingly..