At the moment there is no way to know how much dirty content has been created in cache. We track the amount of currently dirty content, and statistics about writing that dirty content out, but connecting them is inaccurate and hard.

The new statistic could increment a variable in __wt_cache_dirty_incr, similarly to the current WT_CACHE::bytes_dirty_leaf value. That could either be incremented directly into a statistic, or incremented in a new field in the cache and copied across during statistics gathering.