code: func encodeKeyValue(k, v string) (string, string) {
  	k = strings.ToLower(k)
  	if strings.HasSuffix(k, binHdrSuffix) {
  		val := base64.StdEncoding.EncodeToString([]byte(v))
  		v = string(val)
  	}
  	return k, v
  }
docs: // encodeKeyValue encodes key and value qualified for transmission via gRPC.
  // note: copy pasted from private values of grpc.metadata