Add file descriptor usage

[SoyYoRafa]

Hi, Could you please add monitoring for file descriptor usage per instance? It's a resource that can be exhausted and it would be good how much head room there is.

[tzach]

node_exporter expose file descriptor info with filefd[1]

[1] https://github.com/prometheus/node_exporter/blob/master/collector/filefd_linux.go

[amnonh]

The relevant metrics are: node_filefd_allocated and node_filefd_maximum

[SoyYoRafa]

These attributes collect the kernel limits. Can we also collect the scylla process limit ulimit -n? The scylla process would have to export this.

[amnonh]

@tzach is there a point in showing the kernel limit or usage?

[tzach]

Yes, as they might run out, but I'm not sure it's as interesting. @SoyYoRafa please note you can add you own custom metrics

[fee-mendes]

@amnonh bump