Add missing docstring to Prometheus metric

changelogs/unreleased/rp-add-missing-docstring.yml

+---
+title: Add missing docstring to Prometheus metric
+merge_request: 27868
+author:
+type: fixed

lib/gitlab/middleware/rails_queue_duration.rb

@@ -32,6 +32,7 @@ module Gitlab
       def metric_rails_queue_duration_seconds
         @metric_rails_queue_duration_seconds ||= Gitlab::Metrics.histogram(
           :gitlab_rails_queue_duration_seconds,
+          'Measures latency between GitLab Workhorse forwarding a request to Rails',
           Gitlab::Metrics::Transaction::BASE_LABELS
         )
       end

spec/lib/gitlab/middleware/rails_queue_duration_spec.rb

@@ -9,7 +9,7 @@ describe Gitlab::Middleware::RailsQueueDuration do
   let(:transaction) { Gitlab::Metrics::WebTransaction.new(env) }
 
   before do
-    expect(app).to receive(:call).with(env).and_return('yay')
+    allow(app).to receive(:call).with(env).and_return('yay')
   end
 
   describe '#call' do
@@ -42,6 +42,13 @@ describe Gitlab::Middleware::RailsQueueDuration do
           expect(middleware.call(env)).to eq('yay')
         end
       end
+
+      it 'creates a metric with a docstring' do
+        metric = middleware.send(:metric_rails_queue_duration_seconds)
+
+        expect(metric).to be_instance_of(Prometheus::Client::Histogram)
+        expect(metric.docstring).to eq('Measures latency between GitLab Workhorse forwarding a request to Rails')
+      end
     end
   end
 end