message: string stats use reference value in tfstreamz plane.
diff: @@ -100,7 +100,8 @@ Status SerializeToXPlane(const std::vector<TfStreamzSnapshot>& snapshots,
  xevent.AddStatValue(*metadata, point->bool_value);
  break;
  case monitoring::ValueType::kString:
  - xevent.AddStatValue(*metadata, point->string_value);
  + xevent.AddStatValue(*metadata, *xplane.GetOrCreateStatMetadata(
  + point->string_value));
  break;
  case monitoring::ValueType::kHistogram:
  xevent.AddStatValue(*metadata,
  