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

[vundela]

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

[vanzin]

ok to test

[SparkQA]

Test build #83649 has finished for PR 19709 at commit 2a0b281.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[vundela]

retest this please.

[SparkQA]

Test build #83650 has finished for PR 19709 at commit 45925dc.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[vanzin]

LGTM, merging to master.