BUG: Fix filter_func logging for config.debug=true

Fixes #8456

logstash-core/lib/logstash/config/config_ast.rb

@@ -142,7 +142,8 @@ def compile
         definitions << "define_singleton_method :#{type}_func do |event|"
         definitions << "  targeted_outputs = []" if type == "output"
         definitions << "  events = event" if type == "filter"
-        definitions << "  @logger.debug? && @logger.debug(\"#{type} received\", \"event\" => event.to_hash)"
+        definitions << "  @logger.debug? && @logger.debug(\"#{type} received\", \"event\" => event.to_hash)" if type == "output"
+        definitions << "  @logger.debug? && events.each { |e| @logger.debug(\"#{type} received\", \"event\" => e.to_hash)}" if type == "filter"
 
         sections.select { |s| s.plugin_type.text_value == type }.each do |s|
           definitions << s.compile.split("\n", -1).map { |e| "  #{e}" }

logstash-core/spec/logstash/pipeline_spec.rb

@@ -228,6 +228,14 @@ class TestPipeline < LogStash::Pipeline
           pipeline = TestPipeline.new(test_config_with_filters, pipeline_settings_obj)
           pipeline.close
         end
+
+        it "should log each filtered event if config.debug is set to true" do
+          pipeline_settings_obj.set("config.debug", true)
+          pipeline = mock_pipeline_from_string(test_config_with_filters, pipeline_settings_obj)
+          expect(logger).to receive(:debug).with(/filter received/, anything)
+          pipeline.filter_func([LogStash::Event.new])
+          pipeline.close
+        end
       end
 
       context "when there is no command line -w N set" do