Initialization: backport of plugin manager initialization (#31988)

Manual backport of parts of #28377 to make sure services are initialized
in the right order.

pkg/plugins/backendplugin/manager.go

@@ -33,7 +33,11 @@ var (
 )
 
 func init() {
-	registry.RegisterService(&manager{})
+	registry.Register(&registry.Descriptor{
+		Name:         "manager",
+		Instance:     &manager{},
+		InitPriority: registry.MediumHigh,
+	})
 }
 
 // Manager manages backend plugins.

pkg/registry/registry.go

@@ -114,7 +114,8 @@ func IsDisabled(srv Service) bool {
 type Priority int
 
 const (
-	High   Priority = 100
-	Medium Priority = 50
-	Low    Priority = 0
+	High       Priority = 100
+	MediumHigh Priority = 75
+	Medium     Priority = 50
+	Low        Priority = 0
 )