GODRIVER-563 Add latencySelector for writeSelector property in mongo.Database and mongo.Collection

Change-Id: I323d44542822c865e9016be38ca97c35738ca130

Author: Isabella Siu

mongo/collection.go

@@ -74,6 +74,11 @@ func newCollection(db *Database, name string, opts ...collectionopt.Option) *Col
 		description.LatencySelector(db.client.localThreshold),
 	})
 
+	writeSelector := description.CompositeSelector([]description.ServerSelector{
+		description.WriteSelector(),
+		description.LatencySelector(db.client.localThreshold),
+	})
+
 	coll := &Collection{
 		client:         db.client,
 		db:             db,
@@ -82,7 +87,7 @@ func newCollection(db *Database, name string, opts ...collectionopt.Option) *Col
 		readConcern:    rc,
 		writeConcern:   wc,
 		readSelector:   readSelector,
-		writeSelector:  db.writeSelector,
+		writeSelector:  writeSelector,
 		registry:       db.registry,
 	}
 

mongo/database.go

@@ -70,7 +70,10 @@ func newDatabase(client *Client, name string, opts ...dbopt.Option) *Database {
 		description.LatencySelector(db.client.localThreshold),
 	})
 
-	db.writeSelector = description.WriteSelector()
+	db.writeSelector = description.CompositeSelector([]description.ServerSelector{
+		description.WriteSelector(),
+		description.LatencySelector(db.client.localThreshold),
+	})
 
 	return db
 }