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/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