add_host_metadata: populate host.os.type

Adds the host.os.type field introduced by ECS 1.8.0.

Possible values for this field are:
- linux
- macos
- unix
- windows

The field will be missing for OSes not in the list.

Related #23118

CHANGELOG.next.asciidoc

@@ -599,6 +599,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Add kubernetes.volume.fs.used.pct field. {pull}23564[23564]
 - Add the `enable_krb5_fast` flag to the Kafka output to explicitly opt-in to FAST authentication. {pull}23629[23629]
 - Add deployment name in pod's meta. {pull}23610[23610]
+- Added ECS 1.8 `host.os.type` field to `add_host_metadata` processor. {pull}23513[23513]
 
 *Auditbeat*
 

libbeat/metric/system/host/host.go

@@ -53,7 +53,9 @@ func MapHostInfo(info types.HostInfo) common.MapStr {
 	if info.OS.Build != "" {
 		data.Put("host.os.build", info.OS.Build)
 	}
-
+	if info.OS.Type != "" {
+		data.Put("host.os.type", info.OS.Type)
+	}
 	return data
 }
 

libbeat/processors/add_host_metadata/add_host_metadata_test.go

@@ -75,6 +75,10 @@ func TestConfigDefault(t *testing.T) {
 	v, err = newEvent.GetValue("host.mac")
 	assert.NoError(t, err)
 	assert.NotNil(t, v)
+
+	v, err = newEvent.GetValue("host.os.type")
+	assert.NoError(t, err)
+	assert.NotNil(t, v)
 }
 
 func TestConfigNetInfoDisabled(t *testing.T) {
@@ -118,6 +122,10 @@ func TestConfigNetInfoDisabled(t *testing.T) {
 	v, err = newEvent.GetValue("host.mac")
 	assert.Error(t, err)
 	assert.Nil(t, v)
+
+	v, err = newEvent.GetValue("host.os.type")
+	assert.NoError(t, err)
+	assert.NotNil(t, v)
 }
 
 func TestConfigName(t *testing.T) {

libbeat/processors/add_host_metadata/docs/add_host_metadata.asciidoc

@@ -57,6 +57,7 @@ The fields added to the event look like the following:
       "id":"",
       "os":{
          "family":"darwin",
+         "type":"macos",
          "build":"16G1212",
          "platform":"darwin",
          "version":"10.12.6",