MM-21357: Use typed constants ClusterEvent (#17920)

* MM-21357: Use typed constants ClusterEvent

Use typed constants for:
- model.ClusterEvent

https://mattermost.atlassian.net/browse/MM-21357

```release-note
Use typed constant for model.ClusterEvent
```

* trigger CI
```release-note
NONE
```

einterfaces/cluster.go

@@ -12,7 +12,7 @@ type ClusterMessageHandler func(msg *model.ClusterMessage)
 type ClusterInterface interface {
 	StartInterNodeCommunication()
 	StopInterNodeCommunication()
-	RegisterClusterMessageHandler(event string, crm ClusterMessageHandler)
+	RegisterClusterMessageHandler(event model.ClusterEvent, crm ClusterMessageHandler)
 	GetClusterId() string
 	IsLeader() bool
 	// HealthScore returns a number which is indicative of how well an instance is meeting

einterfaces/metrics.go

@@ -5,6 +5,7 @@ package einterfaces
 
 import (
 	"github.com/mattermost/logr"
+	"github.com/mattermost/mattermost-server/v5/model"
 )
 
 type MetricsInterface interface {
@@ -22,7 +23,7 @@ type MetricsInterface interface {
 
 	IncrementClusterRequest()
 	ObserveClusterRequestDuration(elapsed float64)
-	IncrementClusterEventType(eventType string)
+	IncrementClusterEventType(eventType model.ClusterEvent)
 
 	IncrementLogin()
 	IncrementLoginFail()

einterfaces/mocks/ClusterInterface.go

@@ -187,7 +187,7 @@ func (_m *ClusterInterface) NotifyMsg(buf []byte) {
 }
 
 // RegisterClusterMessageHandler provides a mock function with given fields: event, crm
-func (_m *ClusterInterface) RegisterClusterMessageHandler(event string, crm einterfaces.ClusterMessageHandler) {
+func (_m *ClusterInterface) RegisterClusterMessageHandler(event model.ClusterEvent, crm einterfaces.ClusterMessageHandler) {
 	_m.Called(event, crm)
 }
 

einterfaces/mocks/MetricsInterface.go

@@ -7,6 +7,8 @@ package mocks
 import (
 	logr "github.com/mattermost/logr"
 	mock "github.com/stretchr/testify/mock"
+
+	model "github.com/mattermost/mattermost-server/v5/model"
 )
 
 // MetricsInterface is an autogenerated mock type for the MetricsInterface type
@@ -61,7 +63,7 @@ func (_m *MetricsInterface) IncrementChannelIndexCounter() {
 }
 
 // IncrementClusterEventType provides a mock function with given fields: eventType
-func (_m *MetricsInterface) IncrementClusterEventType(eventType string) {
+func (_m *MetricsInterface) IncrementClusterEventType(eventType model.ClusterEvent) {
 	_m.Called(eventType)
 }