#1685 Update ServerCacheFactory to support standard metrics collection

src/main/java/io/ebean/cache/ServerCacheFactory.java

@@ -1,10 +1,19 @@
 package io.ebean.cache;
 
+import io.ebean.meta.MetricVisitor;
+
 /**
  * Defines method for constructing caches for beans and queries.
  */
 public interface ServerCacheFactory {
 
+  /**
+   * Visit the metrics for the cache.
+   */
+  default void visit(MetricVisitor visitor) {
+    // do nothing by default
+  }
+
   /**
    * Create the cache for the given type with options.
    */