lib: replace string concatenation with template

PR-URL: https://github.com/nodejs/node/pull/16917
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

lib/_http_agent.js

@@ -127,10 +127,10 @@ Agent.prototype.getName = function getName(options) {
   // Pacify parallel/test-http-agent-getname by only appending
   // the ':' when options.family is set.
   if (options.family === 4 || options.family === 6)
-    name += ':' + options.family;
+    name += `:${options.family}`;
 
   if (options.socketPath)
-    name += ':' + options.socketPath;
+    name += `:${options.socketPath}`;
 
   return name;
 };