Avoid to increment event.out conter for dropped events (#13593)

Fixes the issue #8752 in event.out counter. When a pipeline contains a drop filter the total out events counter should count only the events that reached the out stage.

This PR changes CompiledExecution.compute() interface to return the number of events that effectively reached the end of the pipeline. This change is used in WorkerLoop to update correctly the event.out metric, instead of relying on the batch's size.

qa/integration/specs/monitoring_api_spec.rb

@@ -17,6 +17,7 @@
 
 require_relative '../framework/fixture'
 require_relative '../framework/settings'
+require_relative '../framework/helpers'
 require_relative '../services/logstash_service'
 require "logstash/devutils/rspec/spec_helper"
 require "stud/try"
@@ -51,6 +52,52 @@ describe "Test Monitoring API" do
     end
   end
 
+  context "verify global event counters" do
+    let(:tcp_port) { random_port }
+    let(:sample_data) { 'Hello World!' }
+    let(:logstash_service) { @fixture.get_service("logstash") }
+
+    before(:each) do
+      logstash_service.spawn_logstash("-w", "1" , "-e", config)
+      logstash_service.wait_for_logstash
+      wait_for_port(tcp_port, 60)
+
+      send_data(tcp_port, sample_data)
+    end
+
+    context "when a drop filter is in the pipeline" do
+      let(:config) { @fixture.config("dropping_events", { :port => tcp_port } ) }
+
+      it 'expose the correct output counter' do
+        try(max_retry) do
+          # node_stats can fail if the stats subsystem isn't ready
+          result = logstash_service.monitoring_api.node_stats rescue nil
+          expect(result).not_to be_nil
+          expect(result["events"]).not_to be_nil
+          expect(result["events"]["in"]).to eq(1)
+          expect(result["events"]["filtered"]).to eq(1)
+          expect(result["events"]["out"]).to eq(0)
+        end
+      end
+    end
+
+    context "when a clone filter is in the pipeline" do
+      let(:config) { @fixture.config("cloning_events", { :port => tcp_port } ) }
+
+      it 'expose the correct output counter' do
+        try(max_retry) do
+          # node_stats can fail if the stats subsystem isn't ready
+          result = logstash_service.monitoring_api.node_stats rescue nil
+          expect(result).not_to be_nil
+          expect(result["events"]).not_to be_nil
+          expect(result["events"]["in"]).to eq(1)
+          expect(result["events"]["filtered"]).to eq(1)
+          expect(result["events"]["out"]).to eq(3)
+        end
+      end
+    end
+  end
+
   it "can retrieve JVM stats" do
     logstash_service = @fixture.get_service("logstash")
     logstash_service.start_with_stdin