grpc-go通过context传递额外数据

• 使用 ctx.Value 从 context 读取数据

// ValueFromIncomingContext returns the metadata value corresponding to the metadata
// key from the incoming metadata if it exists. Key must be lower-case.
//
// # Experimental
//
// Notice: This API is EXPERIMENTAL and may be changed or removed in a
// later release.
func ValueFromIncomingContext(ctx context.Context, key string) []string {
   
   
	md, ok := ctx.Value(mdIncomingKey{
   
   }).(MD)
	if !ok {
   
   
		return nil
	}

	if v, ok := md[key]; ok {