Currently, if an update ends up generating a delta WT_MODIFY instead of a full replacement, the document-bytes-written operation metric will only accrue the count of the modified bytes instead of the entire document byte count. Since the code logic to determine whether to use a delta update or a full update is internal to MongoDB, it is undesirable to leak this internal logic into the public-facing operation metric.

One idea is to set the document-bytes-written metric to the change in size for the document being written, regardless of whether the entire document was replaced or not. The downstream consumers of this metric would have to recognize that this was a delta change and could be negative sometimes.