perf(milestones): refactor (#8788)

# Which Problems Are Solved

Milestones used existing events from a number of aggregates. OIDC
session is one of them. We noticed in load-tests that the reduction of
the oidc_session.added event into the milestone projection is a costly
business with payload based conditionals. A milestone is reached once,
but even then we remain subscribed to the OIDC events. This requires the
projections.current_states to be updated continuously.


# How the Problems Are Solved

The milestone creation is refactored to use dedicated events instead.
The command side decides when a milestone is reached and creates the
reached event once for each milestone when required.

# Additional Changes

In order to prevent reached milestones being created twice, a migration
script is provided. When the old `projections.milestones` table exist,
the state is read from there and `v2` milestone aggregate events are
created, with the original reached and pushed dates.

# Additional Context

- Closes https://github.com/zitadel/zitadel/issues/8800

internal/command/instance_test.go

@@ -13,6 +13,7 @@ import (
 	"golang.org/x/text/language"
 
 	"github.com/zitadel/zitadel/internal/api/authz"
+	"github.com/zitadel/zitadel/internal/cache/noop"
 	"github.com/zitadel/zitadel/internal/command/preparation"
 	"github.com/zitadel/zitadel/internal/crypto"
 	"github.com/zitadel/zitadel/internal/domain"
@@ -20,6 +21,7 @@ import (
 	"github.com/zitadel/zitadel/internal/id"
 	id_mock "github.com/zitadel/zitadel/internal/id/mock"
 	"github.com/zitadel/zitadel/internal/repository/instance"
+	"github.com/zitadel/zitadel/internal/repository/milestone"
 	"github.com/zitadel/zitadel/internal/repository/org"
 	"github.com/zitadel/zitadel/internal/repository/project"
 	"github.com/zitadel/zitadel/internal/repository/user"
@@ -372,6 +374,7 @@ func setupInstanceEvents(ctx context.Context, instanceID, orgID, projectID, appI
 		setupInstanceElementsEvents(ctx, instanceID, instanceName, defaultLanguage),
 		orgEvents(ctx, instanceID, orgID, orgName, projectID, domain, externalSecure, true, true),
 		generatedDomainEvents(ctx, instanceID, orgID, projectID, appID, domain),
+		instanceCreatedMilestoneEvent(ctx, instanceID),
 	)
 }
 
@@ -401,6 +404,12 @@ func generatedDomainEvents(ctx context.Context, instanceID, orgID, projectID, ap
 	}
 }
 
+func instanceCreatedMilestoneEvent(ctx context.Context, instanceID string) []eventstore.Command {
+	return []eventstore.Command{
+		milestone.NewReachedEvent(ctx, milestone.NewInstanceAggregate(instanceID), milestone.InstanceCreated),
+	}
+}
+
 func generatedDomainFilters(instanceID, orgID, projectID, appID, generatedDomain string) []expect {
 	return []expect{
 		expectFilter(),
@@ -1378,7 +1387,7 @@ func TestCommandSide_UpdateInstance(t *testing.T) {
 
 func TestCommandSide_RemoveInstance(t *testing.T) {
 	type fields struct {
-		eventstore *eventstore.Eventstore
+		eventstore func(t *testing.T) *eventstore.Eventstore
 	}
 	type args struct {
 		ctx        context.Context
@@ -1397,8 +1406,7 @@ func TestCommandSide_RemoveInstance(t *testing.T) {
 		{
 			name: "instance not existing, not found error",
 			fields: fields{
-				eventstore: eventstoreExpect(
-					t,
+				eventstore: expectEventstore(
 					expectFilter(),
 				),
 			},
@@ -1413,8 +1421,7 @@ func TestCommandSide_RemoveInstance(t *testing.T) {
 		{
 			name: "instance removed, not found error",
 			fields: fields{
-				eventstore: eventstoreExpect(
-					t,
+				eventstore: expectEventstore(
 					expectFilter(
 						eventFromEventPusher(
 							instance.NewInstanceAddedEvent(
@@ -1444,8 +1451,7 @@ func TestCommandSide_RemoveInstance(t *testing.T) {
 		{
 			name: "instance remove, ok",
 			fields: fields{
-				eventstore: eventstoreExpect(
-					t,
+				eventstore: expectEventstore(
 					expectFilter(
 						eventFromEventPusherWithInstanceID(
 							"INSTANCE",
@@ -1480,6 +1486,10 @@ func TestCommandSide_RemoveInstance(t *testing.T) {
 								"custom.domain",
 							},
 						),
+						milestone.NewReachedEvent(context.Background(),
+							milestone.NewInstanceAggregate("INSTANCE"),
+							milestone.InstanceDeleted,
+						),
 					),
 				),
 			},
@@ -1497,7 +1507,10 @@ func TestCommandSide_RemoveInstance(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			r := &Commands{
-				eventstore: tt.fields.eventstore,
+				eventstore: tt.fields.eventstore(t),
+				caches: &Caches{
+					milestones: noop.NewCache[milestoneIndex, string, *MilestonesReached](),
+				},
 			}
 			got, err := r.RemoveInstance(tt.args.ctx, tt.args.instanceID)
 			if tt.res.err == nil {