code: func (m NiceMD) Add(key string, value string) NiceMD {
  	k, v := encodeKeyValue(key, value)
  	m[k] = append(m[k], v)
  	return m
  }
docs: // Add retrieves a single value from the metadata.
  //
  // It works analogously to http.Header.Add, as it appends to any existing values associated with key.
  //
  // The function is binary-key safe.