Adds regex to match alerts endpoint

To prepare for the addition of support for embedding metrics
tied to an alert in GFM, this adds a regex which can be used
to identify usages of the alerts endpoint.

https://gitlab.com/gitlab-org/gitlab/issues/40997

ee/lib/ee/gitlab/metrics/dashboard/url.rb

@@ -21,6 +21,23 @@ module EE
               )
             end
           end
+
+          # Matches dashboard urls for a metric chart embed
+          # for a specifc firing GitLab alert
+          #
+          # EX - https://<host>/<namespace>/<project>/prometheus/alerts/<alert_id>/metrics_dashboard
+          def alert_regex
+            strong_memoize(:alert_regex) do
+              regex_for_project_metrics(
+                %r{
+                  /prometheus
+                  /alerts
+                  /(?<alert>\d+)
+                  /metrics_dashboard
+                }x
+              )
+            end
+          end
         end
       end
     end

ee/spec/lib/ee/gitlab/metrics/dashboard/url_spec.rb

@@ -31,4 +31,32 @@ describe Gitlab::Metrics::Dashboard::Url do
 
     it_behaves_like 'regex which matches url when expected'
   end
+
+  describe '#alert_regex' do
+    let(:url) do
+      Gitlab::Routing.url_helpers.metrics_dashboard_namespace_project_prometheus_alert_url(
+        'foo',
+        'bar',
+        '1',
+        start: '2020-02-10T12:59:49.938Z',
+        end: '2020-02-10T20:59:49.938Z',
+        anchor: "anchor"
+      )
+    end
+
+    let(:expected_params) do
+      {
+        'url' => url,
+        'namespace' => 'foo',
+        'project' => 'bar',
+        'alert' => '1',
+        'query' => "?end=2020-02-10T20%3A59%3A49.938Z&start=2020-02-10T12%3A59%3A49.938Z",
+        'anchor' => '#anchor'
+      }
+    end
+
+    subject { described_class.alert_regex }
+
+    it_behaves_like 'regex which matches url when expected'
+  end
 end