record web app startup latency

Author: Vladimir Kotal

opengrok-web/src/main/java/org/opengrok/web/WebappListener.java

@@ -56,12 +56,17 @@ public final class WebappListener
         implements ServletContextListener, ServletRequestListener {
 
     private static final Logger LOGGER = LoggerFactory.getLogger(WebappListener.class);
+    private Timer startupTimer = Timer.builder("webapp.startup.latency").
+                description("web application startup latency").
+                register(Metrics.getRegistry());
 
     /**
      * {@inheritDoc}
      */
     @Override
     public void contextInitialized(final ServletContextEvent servletContextEvent) {
+        Instant start = Instant.now();
+
         ServletContext context = servletContextEvent.getServletContext();
         RuntimeEnvironment env = RuntimeEnvironment.getInstance();
 
@@ -97,6 +102,7 @@ public void contextInitialized(final ServletContextEvent servletContextEvent) {
         }
 
         env.startExpirationTimer();
+        startupTimer.record(Duration.between(start, Instant.now()));
     }
 
     /**