Alerting: Add endpoint to revert to a previous alertmanager configuration (#65751)

* Alerting: Add endpoint to revert to a previous alertmanager configuration

This endpoint is meant to be used in conjunction with /api/alertmanager/grafana/config/history to
revert to a previously applied alertmanager configuration. This is done by ID instead of raw config
string in order to avoid secure field complications.

pkg/services/ngalert/store/alertmanager_test.go

@@ -425,6 +425,61 @@ func TestIntegrationGetAppliedConfigurations(t *testing.T) {
 	})
 }
 
+func TestIntegrationGetHistoricalConfiguration(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test")
+	}
+	sqlStore := db.InitTestDB(t)
+	store := &DBstore{
+		SQLStore: sqlStore,
+		Logger:   log.NewNopLogger(),
+	}
+
+	// Tracks the autogenerated PK for the history table.
+	var historyTablePK int64 = 0
+
+	t.Run("no configurations = error", func(tt *testing.T) {
+		_, err := store.GetHistoricalConfiguration(context.Background(), 10, 10)
+		require.Error(tt, err)
+	})
+
+	t.Run("correct configurations should be returned", func(tt *testing.T) {
+		ctx := context.Background()
+		var org int64 = 1
+		setupConfigInOrg(t, "testa", org, store)
+		historyTablePK += 1
+		setupConfigInOrg(t, "testb", org, store)
+		historyTablePK += 1
+
+		cfg, err := store.GetHistoricalConfiguration(ctx, org, historyTablePK)
+		require.NoError(tt, err)
+
+		// Check that the returned configuration is the one that we're expecting.
+		require.Equal(tt, "testb", cfg.AlertmanagerConfiguration)
+	})
+
+	t.Run("configurations from other orgs should not be retrievable by id", func(tt *testing.T) {
+		ctx := context.Background()
+		var org int64 = 1
+		setupConfigInOrg(t, "test1", org, store)
+		historyTablePK += 1
+
+		// Create a config in a different org.
+		var otherOrg int64 = 2
+		setupConfigInOrg(t, "test2", otherOrg, store)
+		historyTablePK += 1
+
+		// Sanity check that config is retrievable with correct org and id.
+		cfg, err := store.GetHistoricalConfiguration(ctx, otherOrg, historyTablePK)
+		require.NoError(tt, err)
+		require.Equal(tt, "test2", cfg.AlertmanagerConfiguration)
+
+		// Verify that we cannot retrieve the config from org=2 when passing in org=1.
+		_, err = store.GetHistoricalConfiguration(ctx, org, historyTablePK)
+		require.Error(tt, err, ErrNoAlertmanagerConfiguration)
+	})
+}
+
 func setupConfig(t *testing.T, config string, store *DBstore) (string, string) {
 	t.Helper()
 	return setupConfigInOrg(t, config, 1, store)