Allow sidekiq_exporter to fail on address in use

The https://gitlab.com/gitlab-org/gitlab/merge_requests/18575
make the exporters to fail hard, where previously it was allowed
to fail softly.

This retains the previous behavior and properly validates that
this is allowed.

This is needed due to lack of mechanism to support shared port
binding when running sidekiq-cluster mode.

lib/gitlab/metrics/exporter/sidekiq_exporter.rb

@@ -14,6 +14,29 @@ module Gitlab
         def log_filename
           File.join(Rails.root, 'log', 'sidekiq_exporter.log')
         end
+
+        private
+
+        # Sidekiq Exporter does not work properly in sidekiq-cluster
+        # mode. It tries to start the service on the same port for
+        # each of the cluster workers, this results in failure
+        # due to duplicate binding.
+        #
+        # For now we ignore this error, as metrics are still "kind of"
+        # valid as they are rendered from shared directory.
+        #
+        # Issue: https://gitlab.com/gitlab-org/gitlab/issues/5714
+        def start_working
+          super
+        rescue Errno::EADDRINUSE => e
+          Sidekiq.logger.error(
+            class: self.class.to_s,
+            message: 'Cannot start sidekiq_exporter',
+            exception: e.message
+          )
+
+          false
+        end
       end
     end
   end

spec/lib/gitlab/metrics/exporter/sidekiq_exporter_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Metrics::Exporter::SidekiqExporter do
+  let(:exporter) { described_class.new }
+
+  after do
+    exporter.stop
+  end
+
+  context 'with valid config' do
+    before do
+      stub_config(
+        monitoring: {
+          sidekiq_exporter: {
+            enabled: true,
+            port: 0,
+            address: '127.0.0.1'
+          }
+        }
+      )
+    end
+
+    it 'does start thread' do
+      expect(exporter.start).not_to be_nil
+    end
+  end
+
+  context 'when port is already taken' do
+    let(:first_exporter) { described_class.new }
+
+    before do
+      stub_config(
+        monitoring: {
+          sidekiq_exporter: {
+            enabled: true,
+            port: 9992,
+            address: '127.0.0.1'
+          }
+        }
+      )
+
+      first_exporter.start
+    end
+
+    after do
+      first_exporter.stop
+    end
+
+    it 'does print error message' do
+      expect(Sidekiq.logger).to receive(:error)
+        .with(
+          class: described_class.to_s,
+          message: 'Cannot start sidekiq_exporter',
+          exception: anything)
+
+      exporter.start
+    end
+
+    it 'does not start thread' do
+      expect(exporter.start).to be_nil
+    end
+  end
+end