How to collect container log on Fluentd

I have installed a microk8s on my Ubuntu 18.04 box and I am pleased with this system.
I set up the fluentd add-on and I can access to the Kibana dashboard.
I see the logs of the system but not the logs of the application pods I have deployed.
The application containers publish app logs on stdout (json docs). I thought it was the default configuration, but app logs are not visible in Kibana.
What shall I do to get the app logs collected by fuentd and visible in Kibana?
Thanks a lot

Hi, Logs appeared in Kibana eventually, I don’t know really why I took so much time.
I need to make sure log rotate. Any clue to do this is welcome.

@geekbot you add kubelet arguments in the file /var/snap/microk8s/current/args/kubelet. These are the arguments you can add regarding log rotation --container-log-max-files and --container-log-max-size.
Check out kubelet reference guide here

Thank you !