Add debug log for determining the number of output workers for output delegator

Fixes #4391

logstash-core/lib/logstash/output_delegator.rb

@@ -31,6 +31,8 @@ module LogStash; class OutputDelegator
     # DO NOT move this statement before the instantiation of the first single instance
     # Read the note above to understand why
     @worker_count = calculate_worker_count(default_worker_count)
+    @logger.debug("Will start workers for output", :worker_count => @worker_count, :class => klass)
+
     warn_on_worker_override!
     # This queue is used to manage sharing across threads
     @worker_queue = SizedQueue.new(@worker_count)

logstash-core/spec/logstash/output_delegator_spec.rb

@@ -115,6 +115,10 @@ describe LogStash::OutputDelegator do
     let(:default_worker_count) { 2 }
     let(:out_klass) { LogStash::Outputs::NOOPDelLegacyNoWorkers }
 
+    before do
+      allow(logger).to receive(:debug).with(any_args)
+    end
+
     it "should only setup one worker" do
       expect(subject.worker_count).to eql(1)
     end