HADOOP-16705. MBeanInfoBuilder puts unnecessary memory pressure on the system with a debug log.

MBeanInfoBuilder's get() method DEBUG logs all the MBeanAttributeInfo attributes that it gathered. This can have a high memory churn that can be easily avoided.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/impl/MBeanInfoBuilder.java

@@ -106,7 +106,7 @@ class MBeanInfoBuilder implements MetricsVisitor {
       }
       ++curRecNo;
     }
-    MetricsSystemImpl.LOG.debug(attrs.toString());
+    MetricsSystemImpl.LOG.debug("{}", attrs);
     MBeanAttributeInfo[] attrsArray = new MBeanAttributeInfo[attrs.size()];
     return new MBeanInfo(name, description, attrs.toArray(attrsArray),
                          null, null, null); // no ops/ctors/notifications