Fails when type `map[string]string` is in proto message

[michael-the-grey]

Error message

cannot use req.Metadata (type map[string]string) as type proto.Message in argument to proto.Marshal:
        map[string]string does not implement proto.Message (missing ProtoMessage method)

Persist Code

	{
		raw, err := proto.Marshal(req.Metadata)
		if err != nil {
			return nil, err
		}
		params.Metadata = raw
	}

Proto Field

  map<string, string> metadata = 11;