feat: add clusterName label for identify each kubernetes cluster where K8UP is deployed

[lenglet-k]

Summary

We use K8up on multiple clusters. These multiple clusters perform backups of namespaces with the same name. All the metrics are sent in the same pushgateway.

As the metrics have the same labels, we cannot have a detailed overview by cluster. This is the way K8UP currently works, which does not allow metric labels to be overloaded. This PR makes it possible to add a clusterName label per Kubernetes cluster.

So in pushgateway we have backup metrics per Kubernetes cluster and per namespace, which improves monitoring of the execution of backups per Kubernetes cluster.

I essentially added a clusterName variable equivalent to the promUrl variable.

See: #1031

Checklist

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• I have not made any changes in the charts/ directory.