@Override
  public T stageOnly() {
    this.delegate.stageOnly();
    return this.self();
  }
Abstract implementation of {@link PendingUpdate}, adding mixed-format logics like tracing and
watermark generating for iceberg operations.

@param <T> Java class of changes from this update; returned by {@link #apply} for validation.
amoro-format-iceberg/src/main/java/org/apache/amoro/op/MixedUpdate.java