chore(useUnifiedTopology): add deprecation warning for useUnifiedTopology

Author: daprahamian

lib/operations/connect.js

@@ -241,6 +241,9 @@ function collectEvents(mongoClient, topology) {
   return collectedEvents;
 }
 
+const emitDeprecationForNonUnifiedTopology = deprecate(() => {},
+'current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. ' + 'To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to MongoClient.connect.');
+
 function connect(mongoClient, url, options, callback) {
   options = Object.assign({}, options);
 
@@ -306,6 +309,8 @@ function connect(mongoClient, url, options, callback) {
       return createTopology(mongoClient, 'unified', _finalOptions, connectCallback);
     }
 
+    emitDeprecationForNonUnifiedTopology();
+
     // Do we have a replicaset then skip discovery and go straight to connectivity
     if (_finalOptions.replicaSet || _finalOptions.rs_name) {
       return createTopology(mongoClient, 'replicaset', _finalOptions, connectCallback);