Preload associations :inverse_of for VimPerformanceState

For Metrics rollup, we fetch the `container_image` from each of the `container` objects.
This change preloads the parent `container_image` value into the `containers` so we avoid the N+1.

before
======

```
container_image = ContainerImage.first
containers = container_image.vim_performance_state(timestamp).containers
containers.map(&:container_image) # N+1
```

after
=====

```
container_image = ContainerImage.first
containers = container_image.vim_performance_state(timestamp).containers
containers.map(&:container_image) # ==> array of container_image object
```

app/models/metric/ci_mixin/state_finders.rb

@@ -37,6 +37,6 @@ def vim_performance_state_association(ts, assoc)
       return respond_to?(:miq_regions) ? miq_regions : []
     end
 
-    vim_performance_state_for_ts(ts).public_send(assoc)
+    vim_performance_state_for_ts(ts).public_send(assoc).tap { |ret| MiqPreloader.preload_reverse(self, assoc, ret) }
   end
 end

lib/miq_preloader.rb

@@ -15,6 +15,23 @@ def self.preload(records, associations, preload_scope = nil)
     preloader.preload(records, associations, preload_scope)
   end
 
+  # If you don't use an association to load records, this method can be used
+  # to assign the parent record to the association
+  #
+  # example:
+  #   vms = Vm.where(:ems_id => ems.id)
+  #   preload_reverse(ems, :ems, vms)
+  #   ==>
+  #   vms.ems is cached
+  #
+  # also of interest:
+  # reverse_association = record.class.reflect_on_association(association).inverse_of.name
+  # ra = value.association(reverse_association) # vms.ems
+  def self.preload_reverse(record, association, values)
+    fa = record.association(association.to_sym)
+    values.each { |value| fa.set_inverse_instance(value) } # do we want to use _from_query?
+  end
+
   # it will load records and their associations, and return the children
   #
   # instead of N+1:

spec/lib/miq_preloader_spec.rb

@@ -40,6 +40,22 @@ def preload(*args)
     end
   end
 
+  describe ".preload_reverse" do
+    it "loads belongs_to in a reverse" do
+      ems = FactoryBot.create(:ems_infra)
+      vms = FactoryBot.create_list(:vm, 2, :ext_management_system => ems)
+
+      vms.each(&:reload)
+      expect(vms.first.association(:ext_management_system)).not_to be_loaded
+      expect { vms.map(&:ext_management_system) }.to make_database_queries
+
+      vms.each(&:reload)
+      expect { MiqPreloader.preload_reverse(ems, :vms, vms) }.not_to make_database_queries
+      expect(vms.first.association(:ext_management_system)).to be_loaded
+      expect { vms.map(&:ext_management_system) }.not_to make_database_queries
+    end
+  end
+
   describe ".preload_and_map" do
     it "preloads from an object" do
       ems = FactoryBot.create(:ems_infra)