Metrics fields now render ad-hoc aggregate/SQL entries as editable rows
via a new AdhocMetricEditor popover instead of falling back to raw JSON
for the whole field whenever any entry isn't a saved-metric string.
Dataset metadata fetch extended with column verbose names and the
disallow_adhoc_metrics flag needed to support it.
Also redesigns the Inspector's Placement section as a collapsed panel
with slider controls, and adds consistent field-row padding across the
panel.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
