add metrics for auth hook rtt

lib/middleware.js

@@ -1,9 +1,22 @@
 const debug = require('debug')('jambonz:sbc-inbound');
+const Emitter = require('events');
+
+class AuthOutcomeReporter extends Emitter {
+  constructor(stats) {
+    super();
+    this.on('regHookOutcome', ({rtt, status}) => {
+      stats.histogram('app.hook.response_time', rtt, ['hook_type:auth', `status:${status}`]);
+    });
+  }
+}
 
 module.exports = function(srf, logger) {
   const {lookupSipGatewayBySignalingAddress, lookupAuthHook}  = srf.locals.dbHelpers;
-  const authenticator = require('jambonz-http-authenticator')(lookupAuthHook, logger, {blacklistUnknownRealms: true});
   const {stats} = srf.locals;
+  const authenticator = require('jambonz-http-authenticator')(lookupAuthHook, logger, {
+    blacklistUnknownRealms: true,
+    emitter: new AuthOutcomeReporter(stats)
+  });
 
   function initLocals(req, res, next) {
     req.locals = req.locals || {};