Alerting: Make Unified Alerting enabled by default for those who do not use legacy alerting (#42200)

* update AlertingEnabled and UnifiedAlertingSettings.Enabled to be pointers
* add a pseudo migration to fix the AlertingEnabled and UnifiedAlertingSettings.Enabled if the latter is not defined
* update the default configuration file to make default value for both 'enabled' flags be undefined

Misc
* update Migrator to expose DB engine. This is needed for a ualert migration to access the database while the list of migrations is created.
* add more verbose failure when migrations do not match

Co-authored-by: gotjosh <josue@grafana.com>
Co-authored-by: Yuriy Tseretyan <yuriy.tseretyan@grafana.com>
Co-authored-by: gillesdemey <gilles.de.mey@gmail.com>

pkg/services/sqlstore/migrator/migrator.go

@@ -5,16 +5,17 @@ import (
 	"time"
 
 	_ "github.com/go-sql-driver/mysql"
-	"github.com/grafana/grafana/pkg/infra/log"
-	"github.com/grafana/grafana/pkg/setting"
-	"github.com/grafana/grafana/pkg/util/errutil"
 	_ "github.com/lib/pq"
 	_ "github.com/mattn/go-sqlite3"
 	"xorm.io/xorm"
+
+	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/setting"
+	"github.com/grafana/grafana/pkg/util/errutil"
 )
 
 type Migrator struct {
-	x          *xorm.Engine
+	DBEngine   *xorm.Engine
 	Dialect    Dialect
 	migrations []Migration
 	Logger     log.Logger
@@ -32,10 +33,10 @@ type MigrationLog struct {
 
 func NewMigrator(engine *xorm.Engine, cfg *setting.Cfg) *Migrator {
 	mg := &Migrator{}
-	mg.x = engine
+	mg.DBEngine = engine
 	mg.Logger = log.New("migrator")
 	mg.migrations = make([]Migration, 0)
-	mg.Dialect = NewDialect(mg.x)
+	mg.Dialect = NewDialect(mg.DBEngine)
 	mg.Cfg = cfg
 	return mg
 }
@@ -49,11 +50,22 @@ func (mg *Migrator) AddMigration(id string, m Migration) {
 	mg.migrations = append(mg.migrations, m)
 }
 
+func (mg *Migrator) GetMigrationIDs(excludeNotLogged bool) []string {
+	result := make([]string, 0, len(mg.migrations))
+	for _, migration := range mg.migrations {
+		if migration.SkipMigrationLog() && excludeNotLogged {
+			continue
+		}
+		result = append(result, migration.Id())
+	}
+	return result
+}
+
 func (mg *Migrator) GetMigrationLog() (map[string]MigrationLog, error) {
 	logMap := make(map[string]MigrationLog)
 	logItems := make([]MigrationLog, 0)
 
-	exists, err := mg.x.IsTableExist(new(MigrationLog))
+	exists, err := mg.DBEngine.IsTableExist(new(MigrationLog))
 	if err != nil {
 		return nil, errutil.Wrap("failed to check table existence", err)
 	}
@@ -61,7 +73,7 @@ func (mg *Migrator) GetMigrationLog() (map[string]MigrationLog, error) {
 		return logMap, nil
 	}
 
-	if err = mg.x.Find(&logItems); err != nil {
+	if err = mg.DBEngine.Find(&logItems); err != nil {
 		return nil, err
 	}
 
@@ -132,7 +144,7 @@ func (mg *Migrator) Start() error {
 	mg.Logger.Info("migrations completed", "performed", migrationsPerformed, "skipped", migrationsSkipped, "duration", time.Since(start))
 
 	// Make sure migrations are synced
-	return mg.x.Sync2()
+	return mg.DBEngine.Sync2()
 }
 
 func (mg *Migrator) exec(m Migration, sess *xorm.Session) error {
@@ -178,7 +190,7 @@ func (mg *Migrator) exec(m Migration, sess *xorm.Session) error {
 type dbTransactionFunc func(sess *xorm.Session) error
 
 func (mg *Migrator) InTransaction(callback dbTransactionFunc) error {
-	sess := mg.x.NewSession()
+	sess := mg.DBEngine.NewSession()
 	defer sess.Close()
 
 	if err := sess.Begin(); err != nil {