[Auditbeat] Start system module without host ID (elastic#12373)

Allows the system module to start without the host ID. It will log a warning, and documents will not contain any `entity_id` fields.

(cherry picked from commit 1328f86)

CHANGELOG.next.asciidoc

@@ -89,6 +89,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix formatting of config files on macOS and Windows. {pull}12148[12148]
 - Fix direction of incoming IPv6 sockets. {pull}12248[12248]
 - Package dataset: Auto-detect package directories. {pull}12289[12289]
+- System module: Start system module without host ID. {pull}12373[12373]
 
 *Filebeat*
 

x-pack/auditbeat/module/system/package/package.go

@@ -345,7 +345,9 @@ func (ms *MetricSet) packageEvent(pkg *Package, eventType string, action eventAc
 		MetricSetFields: pkg.toMapStr(),
 	}
 
-	event.MetricSetFields.Put("entity_id", pkg.entityID(ms.HostID()))
+	if ms.HostID() != "" {
+		event.MetricSetFields.Put("entity_id", pkg.entityID(ms.HostID()))
+	}
 
 	if pkg.Error != nil {
 		event.RootFields.Put("error.message", pkg.Error.Error())

x-pack/auditbeat/module/system/process/process.go

@@ -361,7 +361,9 @@ func (ms *MetricSet) processEvent(process *Process, eventType string, action eve
 		event.RootFields.Put("error.message", process.Error.Error())
 	}
 
-	event.RootFields.Put("process.entity_id", process.entityID(ms.HostID()))
+	if ms.HostID() != "" {
+		event.RootFields.Put("process.entity_id", process.entityID(ms.HostID()))
+	}
 
 	return event
 }

x-pack/auditbeat/module/system/socket/socket.go

@@ -363,7 +363,9 @@ func (ms *MetricSet) socketEvent(socket *Socket, eventType string, action eventA
 	event.RootFields.Put("event.action", action.String())
 	event.RootFields.Put("message", socketMessage(socket, action))
 
-	event.RootFields.Put("socket.entity_id", socket.entityID(ms.HostID()))
+	if ms.HostID() != "" {
+		event.RootFields.Put("socket.entity_id", socket.entityID(ms.HostID()))
+	}
 
 	return event
 }

x-pack/auditbeat/module/system/system.go

@@ -5,15 +5,18 @@
 package system
 
 import (
-	"github.com/pkg/errors"
-
+	"github.com/elastic/beats/libbeat/logp"
 	"github.com/elastic/beats/metricbeat/mb"
 	"github.com/elastic/go-sysinfo"
 )
 
+const (
+	moduleName = "system"
+)
+
 func init() {
 	// Register the custom ModuleFactory function for the system module.
-	if err := mb.Registry.AddModule("system", NewModule); err != nil {
+	if err := mb.Registry.AddModule(moduleName, NewModule); err != nil {
 		panic(err)
 	}
 }
@@ -49,9 +52,16 @@ func NewModule(base mb.BaseModule) (mb.Module, error) {
 		return nil, err
 	}
 
+	log := logp.NewLogger(moduleName)
+
+	var hostID string
 	hostInfo, err := sysinfo.Host()
-	if err != nil {
-		return nil, errors.Wrap(err, "failed to get host ID")
+	if hostInfo != nil {
+		hostID = hostInfo.Info().UniqueID
+	}
+
+	if hostID == "" {
+		log.Warnf("Could not get host ID, will not fill entity_id fields. Error: %+v", err)
 	}
 
 	return &SystemModule{

x-pack/auditbeat/module/system/user/user.go

@@ -427,21 +427,26 @@ func (ms *MetricSet) reportChanges(report mb.ReporterV2) error {
 }
 
 func (ms *MetricSet) userEvent(user *User, eventType string, action eventAction) mb.Event {
-	return mb.Event{
+	event := mb.Event{
 		RootFields: common.MapStr{
 			"event": common.MapStr{
 				"kind":   eventType,
 				"action": action.String(),
 			},
 			"user": common.MapStr{
-				"entity_id": user.entityID(ms.HostID()),
-				"id":        user.UID,
-				"name":      user.Name,
+				"id":   user.UID,
+				"name": user.Name,
 			},
 			"message": userMessage(user, action),
 		},
 		MetricSetFields: user.toMapStr(),
 	}
+
+	if ms.HostID() != "" {
+		event.RootFields.Put("user.entity_id", user.entityID(ms.HostID()))
+	}
+
+	return event
 }
 
 func userMessage(user *User, action eventAction) string {