libbeat/reader: Fix messge conversion to beat.Event (#30057)

`Message.ToEvent` was not copying the `Message.Privage` field when
converting a Message to `beat.Event`. This commit fixes it.

Fixes: #30031

libbeat/reader/message.go

@@ -93,5 +93,6 @@ func (m *Message) ToEvent() beat.Event {
 		Timestamp: m.Ts,
 		Meta:      m.Meta,
 		Fields:    m.Fields,
+		Private:   m.Private,
 	}
 }

libbeat/reader/message_test.go

@@ -19,6 +19,7 @@ package reader
 
 import (
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 
@@ -55,6 +56,20 @@ func TestToEvent(t *testing.T) {
 			Message{Content: []byte("my message"), Fields: common.MapStr{"my_field": "my_value"}, Meta: common.MapStr{"meta": "id"}},
 			beat.Event{Fields: common.MapStr{"message": "my message", "my_field": "my_value"}, Meta: common.MapStr{"meta": "id"}},
 		},
+		"content, meta, message and private fields": {
+			Message{
+				Ts:      time.Date(2022, 1, 9, 10, 42, 0, 0, time.UTC),
+				Content: []byte("my message"),
+				Meta:    common.MapStr{"foo": "bar"},
+				Private: 42,
+			},
+			beat.Event{
+				Timestamp: time.Date(2022, 1, 9, 10, 42, 0, 0, time.UTC),
+				Fields:    common.MapStr{"message": "my message"},
+				Meta:      common.MapStr{"foo": "bar"},
+				Private:   42,
+			},
+		},
 	}
 
 	for name, test := range testCases {