fix removal of NOT NULL constraint

I'm retroactively editing an existing migration because this is only
rolled out on QA so far and thus easy to fix.

internal/collector/capacity_scrape.go

@@ -76,8 +76,8 @@ type capacityScrapeTask struct {
 var (
 	// upsert a cluster_capacitors entry
 	initCapacitorQuery = sqlext.SimplifyWhitespace(`
-		INSERT INTO cluster_capacitors (capacitor_id, scraped_at, next_scrape_at)
-		VALUES ($1, $2, $2)
+		INSERT INTO cluster_capacitors (capacitor_id, next_scrape_at)
+		VALUES ($1, $2)
 		ON CONFLICT DO NOTHING
 	`)
 
@@ -98,6 +98,7 @@ func (c *Collector) discoverCapacityScrapeTask(_ context.Context, _ prometheus.L
 
 	//consistency check: every once in a while (and also immediately on startup),
 	//check that all required `cluster_capacitors` entries exist
+	//(this is important because the query below will only find capacitors that have such an entry)
 	if lastConsistencyCheckAt.Before(task.Timing.StartedAt.Add(-5 * time.Minute)) {
 		err = sqlext.WithPreparedStatement(c.DB, initCapacitorQuery, func(stmt *sql.Stmt) error {
 			for capacitorID := range c.Cluster.CapacityPlugins {

internal/db/migrations.go

@@ -163,7 +163,7 @@ var sqlMigrations = map[string]string{
 	`,
 	"024_move_capacity_scrape_timestamps.up.sql": `
 		ALTER TABLE cluster_capacitors
-			ALTER COLUMN scraped_at SET DEFAULT NULL; -- null if scraping did not happen yet
+			ALTER COLUMN scraped_at DROP NOT NULL; -- null if scraping did not happen yet
 		ALTER TABLE cluster_services
 			DROP COLUMN scraped_at;
 		ALTER TABLE cluster_resources
@@ -175,7 +175,7 @@ var sqlMigrations = map[string]string{
 		ALTER TABLE cluster_services
 			ADD COLUMN scraped_at TIMESTAMP NOT NULL DEFAULT NOW();
 		ALTER TABLE cluster_resources
-			ALTER COLUMN capacitor_id SET DEFAULT NULL;
+			ALTER COLUMN capacitor_id DROP NOT NULL;
 	`,
 	"025_capacity_scan_rework.up.sql": `
 		ALTER TABLE cluster_capacitors