message: [tf.data] Add traceme for prefetch with slack time
diff: @@ -128,7 +128,13 @@ class PrefetchDatasetOp::Dataset : public DatasetBase {
  tf_shared_lock l(mu_);
  buffer_limit = auto_tuner_.buffer_limit();
  }
  - return strings::StrCat(prefix(), "#buffer_limit=", buffer_limit, "#");
  + string prefetch_with_slack_trace = "";
  + if (dataset()->slack_period_ > 0) {
  + int64 slack_us = slack_us_;
  + prefetch_with_slack_trace = strings::StrCat(",slack=", slack_us);
  + }
  + return strings::StrCat(prefix(), "#buffer_limit=", buffer_limit,
  + prefetch_with_slack_trace, "#");
  }
  Status Initialize(IteratorContext* ctx) override {
  