st := status.New(codes.InvalidArgument, "invalid username")
desc := "The username must only contain alphanumeric characters"
v := &errdetails.BadRequest_FieldViolation{
    Field: "username",
    Description: desc,
}
br := &errdetails.BadRequest{}
br.FieldViolations = append(br.FieldViolations, v)
st, err := st.WithDetails(br)
if err != nil {
    // If this errored, it will always error
    // here, so better panic so we can figure
    // out why than have this silently passing.
    panic(fmt.Sprintf("Unexpected error attaching metadata: %v", err))
}
return st.Err()