[SPARK-22483][CORE] Exposing java.nio bufferedPool memory metrics to …

…Metric System

## What changes were proposed in this pull request?

Adds java.nio bufferedPool memory metrics to metrics system which includes both direct and mapped memory.

## How was this patch tested?
Manually tested and checked direct and mapped memory metrics too available in metrics system using Console sink.

Here is the sample console output

application_1509655862825_0016.2.jvm.direct.capacity
             value = 19497
application_1509655862825_0016.2.jvm.direct.count
             value = 6
application_1509655862825_0016.2.jvm.direct.used
             value = 19498

application_1509655862825_0016.2.jvm.mapped.capacity
             value = 0
application_1509655862825_0016.2.jvm.mapped.count
             value = 0
application_1509655862825_0016.2.jvm.mapped.used
             value = 0

Author: Srinivasa Reddy Vundela <vsr@cloudera.com>

Closes #19709 from vundela/SPARK-22483.

core/src/main/scala/org/apache/spark/metrics/source/JvmSource.scala

@@ -17,13 +17,17 @@
 
 package org.apache.spark.metrics.source
 
+import java.lang.management.ManagementFactory
+
 import com.codahale.metrics.MetricRegistry
-import com.codahale.metrics.jvm.{GarbageCollectorMetricSet, MemoryUsageGaugeSet}
+import com.codahale.metrics.jvm.{BufferPoolMetricSet, GarbageCollectorMetricSet, MemoryUsageGaugeSet}
 
 private[spark] class JvmSource extends Source {
   override val sourceName = "jvm"
   override val metricRegistry = new MetricRegistry()
 
   metricRegistry.registerAll(new GarbageCollectorMetricSet)
   metricRegistry.registerAll(new MemoryUsageGaugeSet)
+  metricRegistry.registerAll(
+    new BufferPoolMetricSet(ManagementFactory.getPlatformMBeanServer))
 }