The docs for `GET _nodes/<node>/<metric>` omitted a couple of metrics
and indicated that this API returned dynamic stats rather than static
info. They also didn't mention that `_all` is a legal value, nor
did it give a way to suppress all metrics even though this is possible.
This commit adjusts the docs and adds tests to ensure that selecting
metrics works as expected and to ensure that there is a future-proof
legal way to suppress all metrics.
Closes#79187
Co-authored-by: James Rodewig <40268737+jrodewig@users.noreply.github.com>
5767d51c2b