Adds docs for worker metadata logging

- Previously there was no contributor documentation for
log_extra_metadata_on_done which is a useful helper function in order to
get structured logs out of a workers run.

doc/development/logging.md

@@ -299,6 +299,28 @@ through web requests. See the
 [follow-up work](https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/68)
 for more information.
 
+### Logging context metadata (through workers)
+
+Additional metadata can be attached to a worker through the use of the [`ApplicationWorker#log_extra_metadata_on_done`](https://gitlab.com/gitlab-org/gitlab/-/blob/16ecc33341a3f6b6bebdf78d863c5bce76b040d3/app/workers/concerns/application_worker.rb#L31-34)
+method. Using this method adds metadata that is later logged to Kibana with the done job payload.
+
+```ruby
+class MyExampleWorker
+  include ApplicationWorker
+
+  def perform(*args)
+    # Worker performs work
+    # ...
+    
+    # The contents of value will appear in Kibana under `json.extra.my_example_worker.my_key`
+    log_extra_metadata_on_done(:my_key, value)
+  end
+end
+```
+
+Please see [this example](https://gitlab.com/gitlab-org/gitlab/-/blob/16ecc33341a3f6b6bebdf78d863c5bce76b040d3/app/workers/ci/pipeline_artifacts/expire_artifacts_worker.rb#L20-21)
+which logs a count of how many artifacts are destroyed per run of the `ExpireArtifactsWorker`.
+
 ## Exception Handling
 
 It often happens that you catch the exception and want to track it.