fix: make boundary usage telemetry collection atomic (#21907)

Previously, UpsertBoundaryUsageStats (INSERT...ON CONFLICT DO UPDATE) and
GetAndResetBoundaryUsageSummary (DELETE...RETURNING) could race during
telemetry period cutover. Without serialization, an upsert concurrent with the
delete could lose data (deleted right after being written) or commit after the
delete (miscounted in the next period). Both operations now acquire
LockIDBoundaryUsageStats within a transaction to ensure a clean cutover.

coderd/boundaryusage/tracker.go

@@ -95,15 +95,26 @@ func (t *Tracker) FlushToDB(ctx context.Context, db database.Store, replicaID uu
 	t.mu.Unlock()
 
 	//nolint:gocritic // This is the actual package doing boundary usage tracking.
-	_, err := db.UpsertBoundaryUsageStats(dbauthz.AsBoundaryUsageTracker(ctx), database.UpsertBoundaryUsageStatsParams{
-		ReplicaID:             replicaID,
-		UniqueWorkspacesCount: workspaceCount, // cumulative, for UPDATE
-		UniqueUsersCount:      userCount,      // cumulative, for UPDATE
-		UniqueWorkspacesDelta: workspaceDelta, // delta, for INSERT
-		UniqueUsersDelta:      userDelta,      // delta, for INSERT
-		AllowedRequests:       allowed,
-		DeniedRequests:        denied,
-	})
+	authCtx := dbauthz.AsBoundaryUsageTracker(ctx)
+	err := db.InTx(func(tx database.Store) error {
+		// The advisory lock ensures a clean period cutover by preventing
+		// this upsert from racing with the aggregate+delete in
+		// GetAndResetBoundaryUsageSummary. Without it, upserted data
+		// could be lost or miscounted across periods.
+		if err := tx.AcquireLock(authCtx, database.LockIDBoundaryUsageStats); err != nil {
+			return err
+		}
+		_, err := tx.UpsertBoundaryUsageStats(authCtx, database.UpsertBoundaryUsageStatsParams{
+			ReplicaID:             replicaID,
+			UniqueWorkspacesCount: workspaceCount, // cumulative, for UPDATE
+			UniqueUsersCount:      userCount,      // cumulative, for UPDATE
+			UniqueWorkspacesDelta: workspaceDelta, // delta, for INSERT
+			UniqueUsersDelta:      userDelta,      // delta, for INSERT
+			AllowedRequests:       allowed,
+			DeniedRequests:        denied,
+		})
+		return err
+	}, nil)
 
 	// Always reset cumulative counts to prevent unbounded memory growth (e.g.
 	// if the DB is unreachable). Copy delta maps to preserve any Track() calls