Silence exception when prometheus is unavailable

app/models/clusters/applications/prometheus.rb

@@ -90,7 +90,7 @@ module Clusters
         # ensures headers containing auth data are appended to original k8s client options
         options = kube_client.rest_client.options.merge(headers: kube_client.headers)
         Gitlab::PrometheusClient.new(proxy_url, options)
-      rescue Kubeclient::HttpError, Errno::ECONNRESET, Errno::ECONNREFUSED
+      rescue Kubeclient::HttpError, Errno::ECONNRESET, Errno::ECONNREFUSED, Errno::ENETUNREACH
         # If users have mistakenly set parameters or removed the depended clusters,
         # `proxy_url` could raise an exception because gitlab can not communicate with the cluster.
         # Since `PrometheusAdapter#can_query?` is eargely loaded on environement pages in gitlab,

spec/models/clusters/applications/prometheus_spec.rb

@@ -117,6 +117,12 @@ describe Clusters::Applications::Prometheus do
           let(:exception) { Errno::ECONNRESET }
         end
       end
+
+      context 'when the network is unreachable' do
+        it_behaves_like 'exception caught for prometheus client' do
+          let(:exception) { Errno::ENETUNREACH }
+        end
+      end
     end
   end