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
}
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.
ForeachKey conforms to the TextMapReader interface .