Set default value of prometheus_label to None

[cr7258]

krr calls prometheus.get_label_values function to get a list of all values for the specific label (the default label name is "cluster") to determine whether Prometheus monitors multiple clusters.

However, some metrics may contain the "cluster" label, but it doesn't mean that Prometheus monitors multiple clusters. For example, the cluster label in the following metric refers to the Elasticsearch cluster instead of the Kubernetes cluster.

elasticsearch_cluster_health_status{cluster="elasticsearch-1", color="green", instance="100.96.0.17:9108", job="kubernetes-service-endpoints", kubernetes_node="node-1", namespace="elastic"}

When I run krr simple, I will get this error:

[ERROR] No label specified, Rerun krr with the flag `-l <cluster>` where <cluster> is one of ['elasticsearch-1', 'elasticsearch-2', 'elasticsearch-3']

So I modified the default value of prometheus_label to None in order to prevent multiple cluster checks by default. If the user needs to handle multi-cluster scenarios, they can explicitly specify the "cluster" field.

[CLAassistant]

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[Avi-Robusta]

Great catch, and nice code
One more place I would like you to change before we merge it