RestNodesAction - Should report File Descriptors

[pickypg]

RestNodesAction does not currently report anything related to the number of file descriptors even though it reports other similarly useful metrics, such as heap usage.

• Adding file_desc.current - The current number of file descriptors in use.
• Adding file_desc.percent - The percentage of the maximum number of file descriptors in use (current / max * 100).
• Adding file_desc.max - The maximum number of file descriptors.

In addition, I feel that it would be useful to be able to see the heap.current and ram.current (as ByteSizeValues rather than only being able to see the percentage).