We should add a metrics.document.modified metric to server status, which tracks the number of documents that are modified by update operations. 

We currently have metrics.document.updated - which tracks the number of documents that match the filter criteria of an update operation. 

This can be misleading, as the number of documents which are matched by an update can be different that the number of documents which are modified by an update. 

This new metric can be valuable as a large number of documents matched by an update without being modified may be indicative of the update operations not be sufficiently targeted.