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>

app/busy_test.go

@@ -124,3 +124,4 @@ func (c *ClusterMock) GetPluginStatuses() (model.PluginStatuses, *model.AppError
 func (c *ClusterMock) ConfigChanged(previousConfig *model.Config, newConfig *model.Config, sendToOtherServer bool) *model.AppError {
 	return nil
 }
+func (c *ClusterMock) HealthScore() int { return 0 }

app/server.go

@@ -1242,6 +1242,10 @@ func (s *Server) TotalWebsocketConnections() int {
 	return int(count)
 }
 
+func (s *Server) ClusterHealthScore() int {
+	return s.Cluster.HealthScore()
+}
+
 func (s *Server) ensureDiagnosticId() {
 	if s.diagnosticId != "" {
 		return