feat(upgrade): emit twenty_upgrade_instance_info gauge with version attribute (#20854)

Adds `twenty_upgrade_instance_info` — a new "info"-style gauge that
carries the inferred instance version (derived from the last applied
upgrade migration) as the `version` label.

This follows the standard Prometheus pattern for surfacing string-valued
metadata: value is always `1` (load-bearing for PromQL `group_left`
joins), the data lives on the label. Same shape as `node_uname_info`,
`go_info`, `kube_pod_info`, etc. — see [Prometheus naming
conventions](https://prometheus.io/docs/practices/naming/#metric-names).

On `/metrics`:

```
# HELP twenty_upgrade_instance_info Inferred instance version (semver-ish, derived from the last applied upgrade migration), carried as the `version` attribute
# TYPE twenty_upgrade_instance_info gauge
twenty_upgrade_instance_info{version="2.7.3"} 1
```

Also adds `MetricsService.createInfoGauge` as the helper for the pattern
(auto-suffixes `_info`, enforces value=1). Consumed by
twentyhq/twenty-eng#65.

packages/twenty-server/src/engine/core-modules/metrics/metrics.service.ts

@@ -80,6 +80,40 @@ export class MetricsService {
     return gauge;
   }
 
+  createInfoGauge({
+    metricName,
+    options,
+    attributesCallback,
+  }: {
+    metricName: string;
+    options: MetricOptions;
+    attributesCallback: () => Attributes | Promise<Attributes>;
+  }): ObservableGauge {
+    const normalizedName = metricName.endsWith('_info')
+      ? metricName
+      : `${metricName}_info`;
+
+    const gauge = this.getMeter().createObservableGauge(
+      normalizedName,
+      options,
+    );
+
+    gauge.addCallback(async (observableResult) => {
+      try {
+        const attributes = await attributesCallback();
+
+        observableResult.observe(1, attributes);
+      } catch (error) {
+        this.logger.error(
+          `Failed to collect info gauge ${normalizedName}`,
+          error,
+        );
+      }
+    });
+
+    return gauge;
+  }
+
   async incrementCounterForEvent({
     key,
     eventId,

packages/twenty-server/src/engine/core-modules/upgrade/upgrade-gauge.service.ts

@@ -92,6 +92,22 @@ export class UpgradeGaugeService implements OnModuleInit {
       },
       cacheValue: true,
     });
+
+    this.metricsService.createInfoGauge({
+      metricName: 'twenty_upgrade_instance',
+      options: {
+        description:
+          'Inferred instance version (semver-ish, derived from the last applied upgrade migration), carried as the `version` attribute',
+      },
+      attributesCallback: async () => {
+        const upgradeStatus = await this.getCachedUpgradeStatus();
+
+        return {
+          version:
+            upgradeStatus?.instanceUpgradeStatus.inferredVersion ?? 'unknown',
+        };
+      },
+    });
   }
 
   private async getCachedUpgradeStatus(): Promise<InstanceAndAllWorkspacesUpgradeStatus | null> {