code: func (m metadataTextMap) ForeachKey(callback func(key, val string) error) error {
  	for k, vv := range m {
  		for _, v := range vv {
  			if decodedKey, decodedVal, err := metadata.DecodeKeyValue(k, v); err == nil {
  				if err = callback(decodedKey, decodedVal); err != nil {
  					return err
  				}
  			} else {
  				return fmt.Errorf("failed decoding opentracing from gRPC metadata: %v", err)
  			}
  		}
  	}
  	return nil
  }
docs: // ForeachKey is a opentracing.TextMapReader interface that extracts values.