Secondaries used to insert into oplog using collection_internal::insertDocuments, but SERVER-103421 unified the code paths so that secondaries also call insertDocumentsForOplog the same way as primaries. However, collection_internal::insertDocuments also calls OpObserver's onInserts function, which shouldn't be necessary but they are currently relied upon by unit tests in oplog_application_test, where they register an OpObserver to receive inserted oplog documents and compare with the expected results.