Keep event when add_docker_metadata fails to extract container id

[exekias]

Before this change an error extracting container id from the source path
resulted on lossing the log line.

This change ensures we report the error but still send the log lines
through, without enriching.

Fixes #7129

[ruflin]

Can you add a changelog and will this need to be backported?

[exekias]

I had my doubts about the backport, this should not be a really common case, but backporting should not hurt either, wdyt?

[exekias]

On a second thought, I added backport tags :)

[ruflin]

Backport or not, both works for me :-)

[karmi]

Hi, I'm still getting the lines ignored (not ingested) when there's an error with add_docker_metadata:

2018-07-04T13:20:28.220Z	INFO	[beat]	instance/beat.go:725	Build info	{"system_info": {"build": {"commit": "a04cb664d5fbd4b1aab485d1766f3979c138fd38", "libbeat": "6.3.0", "time": "2018-06-11T22:34:44.000Z", "version": "6.3.0"}}}
...
2018-07-04T13:20:55.295Z	DEBUG	[add_docker_metadata]	add_docker_metadata/add_docker_metadata.go:111	Error while extracting container ID from source path: index is out of range for field 'source'
2018-07-04T13:20:55.297Z	DEBUG	[registrar]	registrar/registrar.go:290	Registry file updated. 1 states written.
2018-07-04T13:20:55.297Z	DEBUG	[registrar]	registrar/registrar.go:232	Processing 1 events
2018-07-04T13:20:55.297Z	DEBUG	[registrar]	registrar/registrar.go:202	Registrar state updates processed. Count: 1
2018-07-04T13:20:55.297Z	DEBUG	[registrar]	registrar/registrar.go:263	Write registry file: /usr/share/filebeat/data/registry
...
2018-07-04T13:21:05.079Z	DEBUG	[input]	log/input.go:168	input states cleaned up. Before: 1, After: 1, Pending: 0
2018-07-04T13:21:10.298Z	DEBUG	[harvester]	log/log.go:85	End of file reached: /var/log/myapp/myapp.log; Backoff now.

[exekias]

Hi @karmi, this fix will go out in filebeat 6.3.1

[karmi]

@exekias, ah right, the v6.3.0 label mislead me...

More importantly, it's an user error on my side :) I've switched from type: docker to type: log (writing from the container to a regular file), so obviously Beats can't infer anything about the Docker environment from the file itself... So apologies for a false alarm!