CacheService: Do not get non existing caches metrics

With the change in the meter implementation, retrieving a non existing
metrics would take time.

For this, the CacheService would mark caches that are not supported with
null url, so the metrics will be register but will return 0 for all
request (instead of going to the API that will return 0).

Signed-off-by: Amnon Heiman <amnon@scylladb.com>

src/main/java/org/apache/cassandra/service/CacheService.java

@@ -65,9 +65,9 @@ public class CacheService implements CacheServiceMBean {
             throw new RuntimeException(e);
         }
 
-        keyCache = new CacheMetrics("KeyCache", "key");
+        keyCache = new CacheMetrics("KeyCache", null);
         rowCache = new CacheMetrics("RowCache", "row");
-        counterCache  = new CacheMetrics("CounterCache", "counter");
+        counterCache  = new CacheMetrics("CounterCache", null);
     }
 
     public int getRowCacheSavePeriodInSeconds() {