Update Prometheus URL spec for 1.10 format

spec/models/clusters/applications/prometheus_spec.rb

@@ -79,7 +79,7 @@ describe Clusters::Applications::Prometheus do
       end
       it 'creates proper url' do
-        expect(subject.prometheus_client.url).to eq('http://example.com/api/v1/proxy/namespaces/gitlab-managed-apps/service/prometheus-prometheus-server:80')
+        expect(subject.prometheus_client.url).to eq('http://example.com/api/v1/namespaces/gitlab-managed-apps/service/prometheus-prometheus-server:80/proxy')
       end
       it 'copies options and headers from kube client to proxy client' do