Exporter: add reservation name as a label to slurm_node_info

[ali-sattari]

Problem

We need to alert when nodes are stuck in particular reservations (done by extensive checks) for too long.

Solution

Adding reservation name that covers each node as a label to existing slurm_node_info metric

Testing

1. Deploy to a test cluster
2. Create reservation using scontrol create reservation nodes=worker-0 reservationname="example reservation" ...
3. Wait for ~30s for scrape, check the metric

Release Notes

Exporter: added reservation_name as a label to slurm_node_info metric.