tracing_keys.go
func (c tracingHeadersCarrier) ForeachKey(handler func(key, val string) error) error {
	for k, v := range c {
		if !strings.HasPrefix(k, tracingKeyPrefix) {
			continue
		}
		noPrefixKey := tracingKeyDecoding.mapAndCache(k)
		if err := handler(noPrefixKey, v); err != nil {
			return err
		}
	}
	return nil
}
func (c tracingHeadersCarrier) ForeachKey(handler func(key, val string) error) error {
	for k, v := range c {
		if !strings.HasPrefix(k, tracingKeyPrefix) {
			continue
		}
		noPrefixKey := tracingKeyDecoding.mapAndCache(k)
		if err := handler(noPrefixKey, v); err != nil {
			return err
		}
	}
	return nil
}
// ForeachKey conforms to the TextMapReader interface.