In dev, always allow access to health endpoints from localhost

This change will allow developers to easily hook up a Prometheus
instance to their local development instance, without making any
configuration changes.

app/controllers/concerns/requires_whitelisted_monitoring_client.rb

@@ -14,6 +14,10 @@ module RequiresWhitelistedMonitoringClient
   end
 
   def client_ip_whitelisted?
+    # Always allow developers to access http://localhost:3000/-/metrics for
+    # debugging purposes
+    return true if Rails.env.development? && request.local?
+
     ip_whitelist.any? { |e| e.include?(Gitlab::RequestContext.client_ip) }
   end
 

changelogs/unreleased/always-allow-prometheus-access-in-dev.yml

@@ -0,0 +1,5 @@
+---
+title: Always allow access to health endpoints from localhost in dev
+merge_request: 29930
+author:
+type: other