Greptile Summary

This PR introduces ordinal as a gauge-only dimension column to distinguish per-replica metric series for multi-node resources (e.g. StatefulSet pods), preventing argMax rows from collapsing across replicas that share the same (metric, resourceId).

• Metric.php: ordinal added to GAUGE_COLUMNS, gauge schema (LowCardinality(Nullable(String))), gauge indexes (set(0) skip index), and a typed getOrdinal(): ?string accessor. Event schema is intentionally unaffected.
• ClickHouse.php: ordinal appended to the lowCardinality list so existing tables gain the column via ADD COLUMN IF NOT EXISTS during setup().
• Database.php: ordinal attribute and index-ordinal appended to the collection definition, but only take effect on first createCollection()—no backfill path exists for existing collections unlike the ClickHouse adapter's ensureDimColumns mechanism.

Confidence Score: 3/5

Safe to merge for ClickHouse deployments; existing Database adapter collections will not receive the new column on upgrade.

The ClickHouse path is well-covered: ensureDimColumns backfills the column on existing tables, live round-trip and schema tests pass, and the index type choice (set(0) on a handful of distinct ordinal values) is appropriate. The gap is in the Database adapter: setup() only calls createCollection(), which is a no-op on an existing collection, so the ordinal attribute and its index are never added. There is no createAttribute/createIndex fallback call analogous to the ClickHouse ADD COLUMN IF NOT EXISTS. Writing a gauge metric with ordinal set against an upgraded Database adapter deployment will either fail or silently drop the field.

src/Usage/Adapter/Database.php — the migration gap for existing collections needs a backfill path comparable to ensureDimColumns.

Important Files Changed

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 2
src/Usage/Adapter/Database.php:117-125
**No migration path for existing Database adapter collections**

The `ordinal` attribute and its index are appended to the `$attributes` / `$indexDocs` arrays before `createCollection()`, but on any deployment where the collection already exists the `DuplicateException` is silently swallowed and neither the attribute nor the index is ever added. The ClickHouse adapter handles this explicitly via `ensureDimColumns` (`ADD COLUMN IF NOT EXISTS`), but there is no equivalent "ensure attribute" call here. Attempting to write a gauge metric with `ordinal` set against an existing collection will either fail or silently drop the field depending on how the underlying utopia-php/database library handles undeclared attributes.

### Issue 2 of 2
tests/Usage/Adapter/ClickHouseSchemaTest.php:114-122
**`ordinal` skip-index assertion missing from gauge schema test**

`testGaugesTableSwapsBloomForSetOnLowCardinality` verifies that `service` and `resourceType` use `set(0)` while `resourceId` and `teamId` use `bloom_filter`, but it does not assert that the new `index-ordinal` was created with `TYPE set(0)`. Adding `assertStringContainsString('`index-ordinal` ordinal TYPE set(0)', $ddl)` would give the same live-DDL coverage that the existing index assertions provide.

_{Reviews (1): Last reviewed commit: "feat: add ordinal gauge dimension column" | Re-trigger Greptile}