add: ContextLogger Interface LogObjectMarshaler (#623)

context.go

@@ -391,6 +391,9 @@ func (c Context) Durs(key string, d []time.Duration) Context {
 
 // Interface adds the field key with obj marshaled using reflection.
 func (c Context) Interface(key string, i interface{}) Context {
+	if obj, ok := i.(LogObjectMarshaler); ok {
+		return c.Object(key, obj)
+	}
 	c.l.context = enc.AppendInterface(enc.AppendKey(c.l.context, key), i)
 	return c
 }

ctx_test.go

@@ -1,6 +1,7 @@
 package zerolog
 
 import (
+	"bytes"
 	"context"
 	"io"
 	"reflect"
@@ -68,3 +69,31 @@ func TestCtxDisabled(t *testing.T) {
 		t.Error("WithContext did not override logger with a disabled logger")
 	}
 }
+
+type logObjectMarshalerImpl struct {
+	name string
+	age  int
+}
+
+func (t logObjectMarshalerImpl) MarshalZerologObject(e *Event) {
+	e.Str("name", "custom_value").Int("age", t.age)
+}
+
+func Test_InterfaceLogObjectMarshaler(t *testing.T) {
+	var buf bytes.Buffer
+	log := New(&buf)
+	ctx := log.WithContext(context.Background())
+
+	log2 := Ctx(ctx)
+
+	withLog := log2.With().Interface("obj", &logObjectMarshalerImpl{
+		name: "foo",
+		age:  29,
+	}).Logger()
+
+	withLog.Info().Msg("test")
+
+	if got, want := buf.String(), `{"level":"info","obj":{"name":"custom_value","age":29},"message":"test"}`+"\n"; got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}