Support kubernetes_role argument for prometheus.operator.servicemonitors

[yoyosir]

PR Description

This PR adds support for endpointslice role in prometheus.operator.servicemonitors. Before this PR, the role defaults to endpoints. So there was no way to have servicemonitor to scrape an endpointslice in alloy.

Example config in alloy after this PR.

prometheus.operator.servicemonitors "servicemonitors_endpointslice" {
  forward_to = [prometheus.relabel.servicemonitors_filter.receiver]
  selector {
    match_expression {
      key      = "kubernetes-scrape-role"
      operator = "In"
      values   = ["endpointslice"]
    }
  }
  clustering {
      enabled = true
  }

  kubernetes_role = "endpointslice"
}

prometheus.operator.servicemonitors "servicemonitors" {
  forward_to = [prometheus.relabel.servicemonitors_filter.receiver]
  selector {
    match_expression {
      key      = "kubernetes-scrape-role"
      operator = "NotIn"
      values   = ["endpointslice"]
    }
  }

  clustering {
      enabled = true
  }

}

Which issue(s) this PR fixes

Fixes #1714

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

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