Surface geo.region as a tag in Discover tag distribution

In Discover, geo.region is available as a column but not as a tag in the tag distribution sidebar. This means users cannot see region breakdowns at a glance, filter by region, or group events by region directly in the UI — they have to export results and analyze externally.

Current behavior

• geo.region (and geo.city, geo.subdivision) exist as column definitions in src/sentry/snuba/events.py and as field keys in the frontend (static/app/utils/fields/index.ts).
• None of these geo fields appear in the tag distribution sidebar in Discover.

Gap

• Users who need to understand regional event distribution must download query results and process them outside Sentry.
• Tag distribution sidebars and faceted filtering are the natural Discover affordance for this use case, but geo context fields are excluded from it.
• The only workaround is to run independent IP geolocation in the SDK and set a custom tag manually — duplicating enrichment Sentry already performs post-ingestion.

Options

• Index geo.region (and related fields) in the tag store so they surface automatically in the tag sidebar.
• Alternatively, explicitly allowlist these fields in the tag distribution endpoint response if they are queryable but not auto-indexed as tags.

geo.country_code would be a natural reference point — if it's already indexed as a tag somewhere, geo.region should follow the same path.

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View Junior Session in Sentry

Start with the geo column definitions in src/sentry/snuba/events.py and the field keys in static/app/utils/fields/index.ts. Compare geo.country_code's existing path to the tag distribution sidebar and response, then determine whether geo.region requires tag-store indexing or endpoint allowlisting. Done means region data appears in Discover's tag distribution and supports filtering or grouping.