message: Make XPlaneBuilder::Id const
diff: @@ -202,7 +202,7 @@ class XPlaneBuilder : public XStatsBuilder<XPlane> {
  public:
  explicit XPlaneBuilder(XPlane* plane);
  - int64 Id() { return plane_->id(); }
  + int64 Id() const { return plane_->id(); }
  void SetId(int64 id) { plane_->set_id(id); }
  void SetName(absl::string_view name) { plane_->set_name(std::string(name)); }
  