fluentd loki output plugin docs: clarify that at least one label must be set explicitly

[jgehrcke]

Here is the documentation for https://github.com/grafana/loki/tree/master/fluentd/fluent-plugin-grafana-loki: https://github.com/grafana/loki/blob/master/docs/clients/fluentd/README.md

It says

In your Fluentd configuration, use @type loki. Additional configuration is optional,

I can't quite confirm that. Looks like you have to set explicitly at least one label.

Labels set on a a previous filter stage are not automatically inherited / used.

When using this:

<match **>
  @type loki
  url "https://logs-us-west1.grafana.net"
</match>

then the plugin sends out the stream fragments with an empty label set, resulting in Loki answering with

400 Bad Request error parsing labels: parse error at line 1, col 2:
syntax error: unexpected }, expecting IDENTIFIER

That is of course a misleading error message. What it means is that the stream object is empty: {} -- no labels set.

Looks like it's not just us falling for that: Also see #1347

I have quickly added a note here: #1909