MM-25404: Add GetHealthScore metric for the cluster (#14767)

* MM-25404: Add GetHealthScore metric for the cluster

This PR adds the necessary function to the cluster interface
so that they can be called from App code.

* Add mocks

* Remove fakeapp

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

einterfaces/cluster.go

@@ -15,6 +15,10 @@ type ClusterInterface interface {
 	RegisterClusterMessageHandler(event string, crm ClusterMessageHandler)
 	GetClusterId() string
 	IsLeader() bool
+	// HealthScore returns a number which is indicative of how well an instance is meeting
+	// the soft real-time requirements of the protocol. Lower numbers are better,
+	// and zero means "totally healthy".
+	HealthScore() int
 	GetMyClusterInfo() *model.ClusterInfo
 	GetClusterInfos() []*model.ClusterInfo
 	SendClusterMessage(cluster *model.ClusterMessage)