Fix a race condition with the add_host_metadata processor (elastic#8223)

Fix a race between updating the host data structure and the
serialization of events to json.

closes elastic#8040

CHANGELOG.asciidoc

@@ -43,6 +43,7 @@ https://github.com/elastic/beats/compare/v6.4.0...master[Check the HEAD diff]
 - Replace index patterns in TSVB visualizations. {pull}7929[7929]
 - Fixed Support `add_docker_metadata` in Windows by identifying systems' path separator. {issue}7797[7797]
 - Add backoff support to x-pack monitoring outputs. {issue}7966[7966]
+- Fix a race condition with the `add_host_metadata` and the event serialization. {pull}8223[8223]
 
 *Auditbeat*
 

libbeat/processors/add_host_metadata/add_host_metadata.go

@@ -70,7 +70,7 @@ func newHostMetadataProcessor(cfg *common.Config) (processors.Processor, error)
 // Run enriches the given event with the host meta data
 func (p *addHostMetadata) Run(event *beat.Event) (*beat.Event, error) {
 	p.loadData()
-	event.Fields.DeepUpdate(p.data)
+	event.Fields.DeepUpdate(p.data.Clone())
 	return event, nil
 }