Add worker label to prometheus metrics

As part of the work to reduce the amount of queues
Sidekiq has to listen to, here we introduce a worker
label that in the future will provide enough information
to query metrics per worker.

Today it's not strictly required given we
have an one-queue-per-worker approach (so we're
able to know which worker is causing issues through
its queue name).

lib/gitlab/sidekiq_middleware/metrics.rb

@@ -9,7 +9,13 @@ module Gitlab
       private
 
       def create_labels(worker_class, queue)
-        labels = { queue: queue.to_s, urgency: "", external_dependencies: FALSE_LABEL, feature_category: "", boundary: "" }
+        labels = { queue: queue.to_s,
+                   worker: worker_class.to_s,
+                   urgency: "",
+                   external_dependencies: FALSE_LABEL,
+                   feature_category: "",
+                   boundary: "" }
+
         return labels unless worker_class && worker_class.include?(WorkerAttributes)
 
         labels[:urgency] = worker_class.get_urgency.to_s

spec/lib/gitlab/sidekiq_middleware/client_metrics_spec.rb

@@ -9,7 +9,14 @@ describe Gitlab::SidekiqMiddleware::ClientMetrics do
     let(:queue) { :test }
     let(:worker_class) { worker.class }
     let(:job) { {} }
-    let(:default_labels) { { queue: queue.to_s, boundary: "", external_dependencies: "no", feature_category: "", urgency: "low" } }
+    let(:default_labels) do
+      { queue: queue.to_s,
+        worker: worker_class.to_s,
+        boundary: "",
+        external_dependencies: "no",
+        feature_category: "",
+        urgency: "low" }
+    end
 
     shared_examples "a metrics client middleware" do
       context "with mocked prometheus" do

spec/lib/gitlab/sidekiq_middleware/server_metrics_spec.rb

@@ -11,7 +11,14 @@ describe Gitlab::SidekiqMiddleware::ServerMetrics do
     let(:job) { {} }
     let(:job_status) { :done }
     let(:labels_with_job_status) { labels.merge(job_status: job_status.to_s) }
-    let(:default_labels) { { queue: queue.to_s, boundary: "", external_dependencies: "no", feature_category: "", urgency: "low" } }
+    let(:default_labels) do
+      { queue: queue.to_s,
+        worker: worker_class.to_s,
+        boundary: "",
+        external_dependencies: "no",
+        feature_category: "",
+        urgency: "low" }
+    end
 
     shared_examples "a metrics middleware" do
       context "with mocked prometheus" do