Turn up the the debug logging on fluent.
But no, I'm not an expert on fluent and now you're having issues connecting from fluent. Perhaps take a look at the docs.
You can exec into the containers and see if the curl still works with the same information you're providing fluent.
Perhaps try without the https? I'm just looking at the docs... The error above doesn't show the actual host it's trying to connect to...
host "elasticsearch-master.monitoring.svc.cluster.local"
If you exec into the container can you do the curl with the same host port etc
That's how I would debug