Creating custom headers | Load Balancing | Google Cloud
Custom request and response headers allow you to specify additional headers that the external HTTP(S) load balancer adds to requests and responses. These headers can include information that the load balancer detects about the client connection, including the latency to the client, the geographic location of the client's IP address, and parameters of the TLS connection.
https://cloud.google.com/load-balancing/docs/custom-headers
