fluent-plugin: Mark as multi-workers ready (#709)

* fluent-plugin: Mark as multi-workers ready

* fluent-plugin: Add info on multi-worker usage to README

fluentd/fluent-plugin-grafana-loki/README.md

@@ -29,6 +29,27 @@ In your Fluentd configuration, use `@type loki`. Additional configuration is opt
 </match>
 ```
 
+### Multi-worker usage
+
+Loki doesn't currently support out-of-order inserts - if you try to insert a log entry an earlier timestamp after a log entry with with identical labels but a later timestamp, the insert will fail with `HTTP status code: 500, message: rpc error: code = Unknown desc = Entry out of order`. Therefore, in order to use this plugin in a multi worker Fluentd setup, you'll need to include the worker ID in the labels.
+
+For example, using [fluent-plugin-record-modifier](https://github.com/repeatedly/fluent-plugin-record-modifier):
+```
+<filter mytag>
+    @type record_modifier
+    <record>
+        fluentd_worker "#{worker_id}"
+    </record>
+</filter>
+
+<match mytag>
+  @type loki
+  # ...
+  label_keys "fluentd_worker"
+  # ...
+</match>
+```
+
 ## Docker Image
 
 There is a Docker image `grafana/fluent-plugin-grafana-loki:master` which contains default configuration files to git log information

fluentd/fluent-plugin-grafana-loki/lib/fluent/plugin/out_loki.rb

@@ -63,6 +63,10 @@ module Fluent
         @label_keys = @label_keys.split(/\s*,\s*/) if @label_keys
       end
 
+      def multi_workers_ready?
+        true
+      end
+
       def http_opts(uri)
         opts = {
           use_ssl: uri.scheme == 'https'