Fix metrics related race condition in sidekiq-cluster

There was a slight chance that the metrics directory got
wiped after workers had started up, which would result
in their metrics missing.

sidekiq_cluster/cli.rb

@@ -100,6 +100,11 @@ def run(argv = ARGV)
 
         unless @dryrun
           @logger.info("Starting cluster with #{queue_groups.length} processes")
+
+          # Make sure we reset the metrics directory prior to:
+          # - starting a metrics server process
+          # - starting new workers
+          ::Prometheus::CleanupMultiprocDirService.new(@metrics_dir).execute
         end
 
         start_and_supervise_workers(queue_groups)
@@ -118,11 +123,6 @@ def start_and_supervise_workers(queue_groups)
 
         return if @dryrun
 
-        # Make sure we reset the metrics directory prior to:
-        # - starting a metrics server process
-        # - starting new workers
-        ::Prometheus::CleanupMultiprocDirService.new(@metrics_dir).execute
-
         ProcessManagement.write_pid(@pid) if @pid
 
         supervisor = SidekiqProcessSupervisor.instance(

spec/commands/sidekiq_cluster/cli_spec.rb

@@ -303,8 +303,9 @@
             allow(Gitlab::SidekiqCluster).to receive(:start).and_return([])
           end
 
-          it 'wipes the metrics directory' do
-            expect(metrics_cleanup_service).to receive(:execute)
+          it 'wipes the metrics directory before starting workers' do
+            expect(metrics_cleanup_service).to receive(:execute).ordered
+            expect(Gitlab::SidekiqCluster).to receive(:start).ordered.and_return([])
 
             cli.run(%w(foo))
           end