Correctly calculate the time spend in the filter + output block

This PR fix an issue where the time was calculated but no work was done
on the event. This code make sure we have at least one event to start
recording the time spend.

This was causing the `events/duratin_in_millis` to not be in sync with
the time spend on the filtera, since `take_batch` was called in a tight
loop and could return an empty array this made the duration was way off.

Fixes: #5952

Fixes #5953

logstash-core/lib/logstash/instrument/metric_store.rb

@@ -188,6 +188,10 @@ module LogStash module Instrument
       end
     end
 
+    def size
+      @fast_lookup.size
+    end
+
     private
     def get_all
       @fast_lookup.values

logstash-core/lib/logstash/util/wrapped_synchronous_queue.rb

@@ -90,9 +90,14 @@ module LogStash; module Util
       def take_batch
         @mutex.synchronize do
           batch = ReadBatch.new(@queue, @batch_size, @wait_for)
-          add_starting_metrics(batch)
           set_current_thread_inflight_batch(batch)
-          start_clock
+
+          # We dont actually have any events to work on so lets
+          # not bother with recording metrics for them
+          if batch.size > 0
+            add_starting_metrics(batch)
+            start_clock
+          end
           batch
         end
       end
@@ -116,8 +121,10 @@ module LogStash; module Util
       end
 
       def stop_clock
-        @inflight_clocks[Thread.current].each(&:stop)
-        @inflight_clocks.delete(Thread.current)
+        unless @inflight_clocks[Thread.current].nil?
+          @inflight_clocks[Thread.current].each(&:stop)
+          @inflight_clocks.delete(Thread.current)
+        end
       end
 
       def add_starting_metrics(batch)

logstash-core/spec/logstash/instrument/metric_store_spec.rb

@@ -203,6 +203,12 @@ describe LogStash::Instrument::MetricStore do
       end
     end
 
+    describe "#size" do
+      it "returns the number of unique metrics" do
+        expect(subject.size).to eq(metric_events.size)
+      end
+    end
+
     describe "#each" do
       it "retrieves all the metric" do
         expect(subject.each.size).to eq(metric_events.size)

logstash-core/spec/logstash/util/wrapped_synchronous_queue_spec.rb

@@ -1,6 +1,7 @@
 # encoding: utf-8
 require "spec_helper"
 require "logstash/util/wrapped_synchronous_queue"
+require "logstash/instrument/collector"
 
 describe LogStash::Util::WrappedSynchronousQueue do
   context "#offer" do
@@ -45,11 +46,47 @@ describe LogStash::Util::WrappedSynchronousQueue do
     end
 
     describe "WriteClient | ReadClient" do
-      context "when writing to the queue" do
-        let(:queue) { DummyQueue.new }
-        let(:write_client) { LogStash::Util::WrappedSynchronousQueue::WriteClient.new(queue)}
-        let(:read_client)  { LogStash::Util::WrappedSynchronousQueue::ReadClient.new(queue)}
+      let(:queue) { DummyQueue.new }
+      let(:write_client) { LogStash::Util::WrappedSynchronousQueue::WriteClient.new(queue)}
+      let(:read_client)  { LogStash::Util::WrappedSynchronousQueue::ReadClient.new(queue)}
 
+      context "when reading from the queue" do
+        let(:collector) { LogStash::Instrument::Collector.new }
+
+        before do
+          read_client.set_events_metric(LogStash::Instrument::Metric.new(collector).namespace(:events))
+          read_client.set_pipeline_metric(LogStash::Instrument::Metric.new(collector).namespace(:pipeline))
+        end
+
+        context "when the queue is empty" do
+          it "doesnt record the `duration_in_millis`" do
+            batch = read_client.take_batch
+            read_client.close_batch(batch)
+            store = collector.snapshot_metric.metric_store
+            expect(store.size).to eq(0)
+          end
+        end
+
+        context "when we have item in the queue" do
+          it "records the `duration_in_millis`" do
+            batch = write_client.get_new_batch
+            5.times {|i| batch.push("value-#{i}")}
+            write_client.push_batch(batch)
+            read_batch = read_client.take_batch
+            sleep(0.1) # simulate some work?
+            read_client.close_batch(batch)
+            store = collector.snapshot_metric.metric_store
+
+            expect(store.size).to eq(4)
+            expect(store.get_shallow(:events, :in).value).to eq(5)
+            expect(store.get_shallow(:events, :duration_in_millis).value).to be > 0
+            expect(store.get_shallow(:pipeline, :in).value).to eq(5)
+            expect(store.get_shallow(:pipeline, :duration_in_millis).value).to be > 0
+          end
+        end
+      end
+
+      context "when writing to the queue" do
         before :each do
           read_client.set_events_metric(LogStash::Instrument::NullMetric.new)
           read_client.set_pipeline_metric(LogStash::Instrument::NullMetric.new)