Add file descriptor details to cat/nodes

cat/nodes currently does not report any details related to file descriptors. This adds the current number in use, the maximum number available as well as their ratio (percentage) to cat/nodes as hidden-by-default metrics. In addition, this also adds current heap usage (as a non-percentage of ts max) and ram usage (as a non-percerntage of its max) to allow tools to provide more granularity.

While adding a test for the default headers, I noticed that heap.percent and ram.percent were actually displayed by default, so I updated their documentation.

Closes #7652