How to create td-agent pod in k8s cluster using juju charm
There’s no quick answer here 
It’s hard to answer without knowing how familiar you are already with Juju and charming.
There’s some existing documentation / information on writing k8s charms, eg
plus a number of other posts in the Kubernetes category on advanced configuration, storage etc
Here’s there’s a generic charm writing guide:
Note though that there’s also a charm operator framework which is being used to write charms moving forward:
https://github.com/canonical/operator
Hopefully some of the above links are useful. Of course, you will also need to know about fluentd and how that all needs to be set up and configured.