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)
- New `connectors/openmetadata/transformer.py` with shared parsing logic
for extracting categories, grain, dimensions, expressions from OM tags
- New `src/catalog_export.py` script (python -m src.catalog_export) that
fetches metrics/tables from OpenMetadata API and writes YAML files to
/data/docs/metrics/ and /data/docs/tables/ for agent consumption
- Refactor webapp/app.py to delegate to transformer (with inline fallback)
- Add `fields` parameter to client.get_metrics() and get_metric_by_fqn()
for fetching tags+owners in a single API call
- Fix pre-existing mock bug in test_openmetadata_enricher (base_url)
- 101 new tests (80 transformer + 21 export), all passing
