HBASE-3680 Publish more metrics about mslab

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1196232 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/hadoop/hbase/regionserver/MemStore.java

@@ -136,6 +136,18 @@ public class MemStore implements HeapSize {
     }
   }
 
+  /**
+   * @return the number of bytes "wasted" by external fragmentation
+   * in the MSLAB, if configured.
+   */
+  long getMslabWaste() {
+    if (allocator != null) {
+      return allocator.getWastedBytes();
+    } else {
+      return 0;
+    }
+  }
+
   void dump() {
     for (KeyValue kv: this.kvset) {
       LOG.info(kv);