docker-driver: loki-relabel-config does not remove the source label

[rgl]

Describe the bug

loki-relabel-config does not remove the source label

To Reproduce

Steps to reproduce the behavior:

1. Start Loki 2.2.1
2. Install the docker-driver 2.2.1 docker plugin
3. Set the following docker 20.10.7 daemon configuration:

   {
     "debug": true,
     "log-driver": "loki",
     "log-opts": {
       "loki-url": "http://10.10.0.2:3100/loki/api/v1/push",
       "loki-external-labels": "job=docker",
       "loki-relabel-config": "- regex: filename|source\n  action: labeldrop\n"
     }
   }

4. Query: logcli query --tail '{job="docker"}'

Got:

2021-06-07T08:09:27+01:00 {container_name="hello-docker", host="linuxkit-52540080b679", job="docker", source="stdout"} hello docker Mon Jun 7 07:09:27 UTC 2021

Expected behavior

I expected the source label to be removed.

Please note that the filename label was removed as expected.

[cyriltovena]

The source label is actually always added if it has a value from docker. To remove it you need to use pipeline stages, which are run at last on every lines.

The relabeling phase is happening only once per container and it is applied on the container metadata once it starts, at this point the label doesn't yet exist.

[rgl]

Thank you for clarifying this for me! It all makes sense now :-)

Will try to use the pipeline stage.