SQL-exporter: collect cratedb_expected_data_nodes metric

[goat-ssh]

Summary of changes

Checklist

• Link to issue this PR refers to: https://github.com/crate/cloud/issues/2154
• Relevant changes are reflected in CHANGES.rst
• Added or changed code is covered by tests
• Documentation has been updated if necessary
• Changed code does not contain any breaking changes (or this is a major version change)

[goat-ssh]

@WalBeh do you think this interval is good? I'm thinking about alert like:

  - alert: CrateDBExpectedNodesStatefulSetMismatch
    # This alert fires when the number of Kubernetes StatefulSet replicas is less than the expected CrateDB nodes
    expr: |
      max by (environment, provider, region, namespace) (cratedb_expected_data_nodes) 
      > 
      max by (environment, provider, region, exported_namespace) (kube_statefulset_replicas{statefulset=~"crate-data-hot-.*"})
    for: 5m