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>

testlib/cluster.go

@@ -21,6 +21,10 @@ func (c *FakeClusterInterface) RegisterClusterMessageHandler(event string, crm e
 	c.clusterMessageHandler = crm
 }
 
+func (c *FakeClusterInterface) HealthScore() int {
+	return 0
+}
+
 func (c *FakeClusterInterface) GetClusterId() string { return "" }
 
 func (c *FakeClusterInterface) IsLeader() bool { return false }