Add jvm.file_descriptor.count metric to the instrumentation/runtime-telemetry

[schmikei]

Is your feature request related to a problem? Please describe.

This is a follow-up of #13722, where we added a metric to jvm.yaml but its unused in the instrumentation/runtime-telemetry.

#13722 (comment)

Opening this PR so that this issue is not lost

Describe the solution you'd like

Feel free to assign myself to this issue. I will try to get to it over the next week!

Describe alternatives you've considered

No response

Additional context

No response