Leverage user-defined request headers

[imjasonh]

https://cloud.google.com/compute/docs/load-balancing/http/backend-service#user-defined-request-headers

By configuring these request headers and echoing them in the response, we should be able to show in the UI where the load balancer thinks you are.

This would require a fairly hefty rewrite of the ping mechanism though, which currently relies on loading an <img> to perform HTTP requests, since IIRC using XMLHttpRequest had more overhead for some reason (though it would contain response headers).