fix(logging): inconsistency between go kit Logger and spanLogger

Only log.Valuer passed via log.With are treated as dynamic argument in go kit log. The change makes spanLogger consistent with go kit.

logging/log.go

@@ -106,14 +106,14 @@ type spanLogger struct {
 }
 
 func (s spanLogger) Log(keyvals ...interface{}) error {
-	s.kvs = append(s.kvs, keyvals...)
 	for k := range s.kvs {
 		if f, ok := s.kvs[k].(log.Valuer); ok {
 			s.kvs[k] = f()
 		}
 	}
+	s.kvs = append(s.kvs, keyvals...)
 	s.span.LogKV(s.kvs...)
-	return s.base.Log(s.kvs...)
+	return s.base.Log(keyvals...)
 }
 
 // WithContext decorates the log.Logger with information form context. If there is an opentracing span
@@ -127,9 +127,11 @@ func WithContext(logger log.Logger, ctx context.Context) log.Logger {
 		args = append(args, kv.Key, kv.Val)
 	}
 
+	base := log.With(logger, args...)
+
 	span := opentracing.SpanFromContext(ctx)
 	if span == nil {
-		return withContext(logger, ctx)
+		return base
 	}
 	return spanLogger{span: span, base: logger, kvs: args}
 }

logging/log_test.go

@@ -65,8 +65,8 @@ func TestSpanLogger(t *testing.T) {
 	spanLogger{
 		span: &mock,
 		base: log.NewNopLogger(),
-		kvs:  []interface{}{"foo", "bar"},
-	}.Log("baz", log.Valuer(func() interface{} { return "qux" }))
+		kvs:  []interface{}{"foo", log.Valuer(func() interface{} { return "bar" })},
+	}.Log("baz", "qux")
 
 	assert.Equal(t, []interface{}{"foo", "bar", "baz", "qux"}, mock.received)
 }