Geo: Fix metrics updates when secondary nodes are cached

Due to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8802,
`Gitlab::Geo.secondary_nodes` can return an Array instead of an
ActiveRecord relation if the value is retrieved from the cache.  Fix
this by using `each` instead of `find_each`, since loading a few GeoNode
records into memory shouldn't be a big deal.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9000

ee/app/services/geo/metrics_update_service.rb

@@ -14,7 +14,7 @@ module Geo
       update_prometheus_metrics(current_node, current_node_status) if prometheus_enabled?
 
       if Gitlab::Geo.primary? && prometheus_enabled?
-        Gitlab::Geo.secondary_nodes.find_each { |node| update_prometheus_metrics(node, node.status) }
+        Gitlab::Geo.secondary_nodes.each { |node| update_prometheus_metrics(node, node.status) }
       end
     end
 

ee/spec/services/geo/metrics_update_service_spec.rb

@@ -124,6 +124,16 @@ describe Geo::MetricsUpdateService, :geo, :prometheus do
       it 'updates the GeoNodeStatus entry' do
         expect { subject.execute }.to change { GeoNodeStatus.count }.by(1)
       end
+
+      it 'updates metrics when secondary nodes are cached', :request_store do
+        allow(subject).to receive(:update_prometheus_metrics).and_call_original
+        expect(subject).to receive(:update_prometheus_metrics).with(secondary, anything).twice
+        expect(subject).to receive(:update_prometheus_metrics).with(another_secondary, anything).twice
+
+        2.times do
+          subject.execute
+        end
+      end
     end
 
     context 'when node is a secondary' do