Stackdriver uptime checks mark legacy services unhealthy. What immediate action should they take?

Uptime checks failing for legacy services often means the monitoring agents can't reach the services due to network restrictions. The most direct solution is to allow traffic from the uptime check servers. Downloading the list of IP addresses for Stackdriver uptime checkers and creating an inbound firewall rule to permit these IPs will immediately resolve the connectivity issue. Installing the Stackdriver agent is not applicable here as uptime checks are external probes, not agent-based monitoring. Configuring the load balancer or web servers to pass or check the User-Agent header is a more complex and less reliable method for allowing access, as User-Agent strings can be spoofed or change, and it doesn't address fundamental network blocking.