Describe the bug
Currently, there is no way to catch if a request to Loki has failed on fluentD side.
Usually, fluentD plugins should adopt https://docs.fluentd.org/plugin-helper-overview/api-plugin-helper-event_emitter, and then if an error is happening it will go to label @error. From then on a Prometheus metrics could be exposed or another action could be taken

More info on error label: https://docs.fluentd.org/configuration/config-file#error-label

To Reproduce
Steps to reproduce the behavior:

1. Create a message that fails to be sent to Loki for some reason
2. Check that the ERROR label configuration in FluentD is not triggered and there is no way to take actions

Expected behavior
Whenever an error is caused while sending logs to Loki an event emitter should be used to propagate the error so that It could be handled