Add filter_tag support to catalog_export and webapp so only metrics
with the required tag are exported to YAML and displayed in UI.
Previously all 19+ metrics were exported regardless of relevance.
- Add has_tag() helper to transformer module
- catalog_export.py: filter_tag parameter from instance.yaml openmetadata config
- webapp/app.py: filter metrics in _load_metrics_from_catalog()
- 7 new tests (has_tag, filter_tag export, stale cleanup)
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