[prometheus-nut-exporter] Set `instance` label instead of `target`.

[reitermarkus]

Description of the change

As far as I understand, Grafana groups by instance by default, so each NUT server should be in instance, otherwise instance will change with every pod rescheduling. This results in UPSs not being grouped when they should be.

Benefits

This makes the example Grafana dashboard work by default.

Possible drawbacks

Breaking change needs major version bump. Maybe I'm also understanding something wrong, since no chart in this repo currently replaces the instance label.

Applicable issues

N/A

Additional information

Checklist

• [x] Title of the PR starts with chart name (e.g. [home-assistant])
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] Chart artifacthub.io/changes changelog annotation has been updated in Chart.yaml. See Artifact Hub documentation for more info.
• [ ] Variables have been documented in the values.yaml file.

[reitermarkus]

I guess I misunderstood, since the pod label also changes and causes it to not group correctly.