propagation.go
func (c TextMapCarrier) ForeachKey(handler func(key, val string) error) error {
	for k, v := range c {
		if err := handler(k, v); err != nil {
			return err
		}
	}
	return nil
}
func (c TextMapCarrier) ForeachKey(handler func(key, val string) error) error {
	for k, v := range c {
		if err := handler(k, v); err != nil {
			return err
		}
	}
	return nil
}
// ForeachKey conforms to the TextMapReader interface.