A bit of documentation.

documentation/developer/Performance-Metrics.md

@@ -0,0 +1,18 @@
+
+The Libresonic framework contains a convenient class (called MetricsManager) to add inner metrics that constructs in real time some performance indicators.
+
+The use of MetricsManager is illustrated in the org.libresonic.player.filter.MetricsFilter class. 
+
+The MetricsFilter adds a metric based on the time spent by each /main.view HTTP request. This is interesting as the main.view request is invoqued when something is displayed in the main Libresonic web frame.
+
+By default, the MetricsManager is deactivated; it does nothing. 
+It can be activated only by adding the following line inside the livresonic.properties configuration file :
+
+```
+Metrics=
+```
+
+Once the MetricsManager as been activated this way, each metric can be inspected using a jmx console like VisualVM.
+Each metric is registered as a MBean as shown below.
+
+![](metrics-visualvm-screenshot.png)

libresonic-main/src/main/java/org/libresonic/player/filter/MetricsFilter.java

@@ -25,6 +25,7 @@ public class MetricsFilter implements Filter {
         HttpServletRequest httpServletRequest = (HttpServletRequest)request;
 
         String timerName = httpServletRequest.getRequestURI();
+        // Add a metric that measures the time spent for each http request for the /main.view url.
         try (MetricsManager.Timer t = metricsManager.condition(timerName.contains("main.view")).timer(this,timerName)) {
             chain.doFilter(request, response);
         }