Databricks converter drops `display_name` on measures during round-trip

[GMorrisXBL]

A Metric View measure with a display_name loses it after import. The field is silently dropped on import and never restored on export.

Steps to reproduce

1. Create a Metric View YAML with a measure that has display_name:

   version: "1.1"
   source: catalog.schema.fact_table
   measures:
     - name: total_revenue
       expr: SUM(amount)
       display_name: Total Revenue

2. Import: ossie-databricks import -i view.yaml -o model.yaml
3. Export: ossie-databricks export -i model.yaml -o view_out.yaml
4. Observe display_name is absent from view_out.yaml.

Root cause

display_name is handled for dimensions but not measures.

Import (metric_view_to_ossie.py, _convert_measure): display_name is never read — compare with _convert_dimension which maps it to the label field.

Export (ossie_to_metric_view.py, _convert_metric): label is never written back to display_name — compare with _convert_field which does this correctly.

Additionally, label is not part of the Metric schema (only Field has it), so display_name on a measure cannot map to a native Ossie field. It should be stashed in custom_extensions[DATABRICKS] (same pattern as format/window) so it survives the round-trip.

Expected behaviour

display_name on a measure is preserved after import → export.

Proposed fix

1. Add display_name to a measure-specific stash key tuple in metric_view_to_ossie.py (separate from _COLUMN_STASH_KEYS to avoid double-stashing it for dimensions, where it already maps to label).
2. Read it back from stash in _convert_metric in ossie_to_metric_view.py and emit display_name.