I have a question about deletion vector metrics. In some of the test files in the master branch, I see metrics for Added and Removed:

"operationMetrics": {
      "numDeletionVectorsAdded": "1",
      "numDeletionVectorsRemoved": "0",

I cannot seem to find any references to

numDeletionVectorsAdded

in the source code, so I am wondering where there is defined?