The 'fastmodinsert' metric reported in the profiler and slow diagnostic log for CRUD operations was deprecated under SERVER-23272 / DOCS-7697 and will be removed under this ticket.

fastmodinsert tracks update operations that result in an insert, given the update is structured as a document replacement, rather than modification. While at one point of time it is possible that this was useful information (questionable given it is hard to find reference to it providing value in either public or internal posts), it is no longer used in a meaningful way. It is not clear that a distinction should be made between a replacement style update and a modifier update that result in an insert or that one is necessarily more expensive than the other.