fix: implement fake DeleteOldWorkspaceAgentStats (#11076)

coderd/database/dbmem/dbmem.go

@@ -1180,8 +1180,22 @@ func (q *FakeQuerier) DeleteOldWorkspaceAgentLogs(_ context.Context) error {
 	return nil
 }
 
-func (*FakeQuerier) DeleteOldWorkspaceAgentStats(_ context.Context) error {
-	// no-op
+func (q *FakeQuerier) DeleteOldWorkspaceAgentStats(_ context.Context) error {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+
+	now := dbtime.Now()
+	sixMonthInterval := 6 * 30 * 24 * time.Hour
+	sixMonthsAgo := now.Add(-sixMonthInterval)
+
+	var validStats []database.WorkspaceAgentStat
+	for _, stat := range q.workspaceAgentStats {
+		if stat.CreatedAt.Before(sixMonthsAgo) {
+			continue
+		}
+		validStats = append(validStats, stat)
+	}
+	q.workspaceAgentStats = validStats
 	return nil
 }