Add Geo node status metrics

Summary

• Add an opt-in database prober for persisted Geo node status metrics.
• Match Rails Geo metric names and {name, url} labels; limit output to Rails-exported status metrics and exclude usage-data counters.
• Document the probe configuration.

Related to gitlab-org/gitlab#590761 (closed)

Validation

• bundle exec rspec spec/database/geo_node_status_spec.rb: 4 examples, 0 failures.
• bundle exec rubocop lib/gitlab_exporter/database/geo_node_status.rb spec/database/geo_node_status_spec.rb: no offenses.
• Full bundle exec rspec spec: 107 examples; 3 unrelated Git push specs fail because temporary remotes reject updates to checked-out branches.