Fix metrics server not shutting down when Puma is stopped

Previously if a separate metrics server were configured and
Puma were stopped, the metrics server would continue to run. When
Puma is started again, it attempts to spawn a new metrics server,
but this fails repeatedly because the port has already been taken.

To fix this, add an `at_exit` handler that will shutdown the process
supervisor if Puma exists. This won't help the case if Puma is
abruptly killed, however.

Relates to https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/8109

Changelog: fixed

metrics_server/metrics_server.rb

@@ -17,6 +17,12 @@ def start_for_puma
       end
 
       supervisor = PumaProcessSupervisor.instance
+
+      at_exit do
+        Gitlab::AppLogger.info("Puma process #{Process.pid} is exiting, shutting down metrics server...")
+        supervisor.shutdown
+      end
+
       supervisor.supervise(start_server.call) do
         Gitlab::AppLogger.info('Puma metrics server terminated, restarting...')
         start_server.call