fix unknown prefix handling, add /js

Author: Vladimir Kotal

opengrok-indexer/src/main/java/org/opengrok/indexer/web/Prefix.java

@@ -79,7 +79,9 @@ public enum Prefix {
     /** Monitoring. */
     METRICS("/metrics"),
     /** CSS and images. */
-    STATIC("/default");
+    STATIC("/default"),
+    /** JavaScript. */
+    JS("/js");
 
     private final String prefix;
     Prefix(String prefix) {

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

@@ -80,7 +80,12 @@ private void measure(HttpServletResponse httpResponse, HttpServletRequest httpRe
         if (isRoot(httpReq)) {
             category = "root";
         } else {
-            category = config.getPrefix().toString().substring(1);
+            String prefix = config.getPrefix().toString();
+            if (prefix.isEmpty()) {
+                category = "unknown";
+            } else {
+                category = prefix.substring(1);
+            }
         }
 
         Timer categoryTimer = Timer.builder("requests.latency").