//      non-NULL, in which case it must point to a `TF_Buffer` containing the
//      serialized representation of a `RunOptions` protocol buffer.
//    - `run_metadata` may be NULL, in which case it will be ignored; or
//      non-NULL, in which case it must point to an empty, freshly allocated
//      `TF_Buffer` that may be updated to contain the serialized representation
//      of a `RunMetadata` protocol buffer.
//