added db connection log message

src/app.ts

@@ -31,6 +31,7 @@ const app = express();
 // Connect to MongoDB
 const mongoUrl = MONGODB_URI;
 (<any>mongoose).Promise = bluebird;
+logger.debug(`Connecting to database with connection string: ${mongoUrl}`);
 mongoose.connect(mongoUrl, {useMongoClient: true}).then(
   () => { /** ready to use. The `mongoose.connect()` promise resolves to undefined. */ },
 ).catch(err => {