Ignore Puma empty worker stats

In some cases (during worker start) it's possible that
Puma.stats returns an empty hash for worker's last status. In
that case we just skip sampling of the worker until these
stats are available.

lib/gitlab/metrics/samplers/puma_sampler.rb

@@ -51,10 +51,11 @@ module Gitlab
           set_master_metrics(stats)
 
           stats['worker_status'].each do |worker|
+            last_status = worker['last_status']
             labels = { worker: "worker_#{worker['index']}" }
 
             metrics[:puma_phase].set(labels, worker['phase'])
-            set_worker_metrics(worker['last_status'], labels)
+            set_worker_metrics(last_status, labels) if last_status.present?
           end
         end
 

spec/lib/gitlab/metrics/samplers/puma_sampler_spec.rb

@@ -61,6 +61,33 @@ describe Gitlab::Metrics::Samplers::PumaSampler do
       end
     end
 
+    context 'with empty worker stats' do
+      let(:puma_stats) do
+        <<~EOS
+        {
+          "workers": 2,
+          "phase": 2,
+          "booted_workers": 2,
+          "old_workers": 0,
+          "worker_status": [{
+            "pid": 32534,
+            "index": 0,
+            "phase": 1,
+            "booted": true,
+            "last_checkin": "2019-05-15T07:57:55Z",
+            "last_status": {}
+          }]
+        }
+        EOS
+      end
+
+      it 'does not log worker stats' do
+        expect(subject).not_to receive(:set_worker_metrics)
+
+        subject.sample
+      end
+    end
+
     context 'in single mode' do
       let(:puma_stats) do
         <<~EOS