[Auditbeat] Add ECS categories for system-host module (#18031)

* [Auditbeat] Add ECS categories for system-host module

* Add changelog entry

* Add host.name

* Add host.name to ECS-only fields

* Remove added 'name' field

CHANGELOG.next.asciidoc

@@ -229,6 +229,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix memory leak on when we miss socket close kprobe events. {pull}17500[17500]
 - Add system module process dataset ECS categorization fields. {pull}18032[18032]
 - Add system module socket dataset ECS categorization fields. {pull}18036[18036]
+- Add ECS categories for system module host dataset. {pull}18031[18031]
 
 *Filebeat*
 

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

@@ -67,6 +67,23 @@ func (action eventAction) String() string {
 	}
 }
 
+func (action eventAction) Type() string {
+	switch action {
+	case eventActionHost:
+		return "info"
+	case eventActionIDChanged:
+		return "change"
+	case eventActionReboot:
+		return "start"
+	case eventActionHostnameChanged:
+		return "change"
+	case eventActionHostChanged:
+		return "change"
+	default:
+		return "info"
+	}
+}
+
 // Host represents information about a host.
 type Host struct {
 	Info types.HostInfo
@@ -322,8 +339,10 @@ func hostEvent(host *Host, eventType string, action eventAction) mb.Event {
 	event := mb.Event{
 		RootFields: common.MapStr{
 			"event": common.MapStr{
-				"kind":   eventType,
-				"action": action.String(),
+				"kind":     eventType,
+				"category": []string{"host"},
+				"type":     []string{action.Type()},
+				"action":   action.String(),
 			},
 			"message": hostMessage(host, action),
 		},