Send Proxy Protocol to Upstream over L4

[apsega]

Title: Send Proxy Protocol to Upstream over L4

Description:
I couldn't find that Envoy Proxy supports sending Proxy protocol to upstreams over L4. This is needed if I have Nginx Ingress Controller under NAT and Envoy Proxy as an edge L4 proxy in front of Kubernetes clusters. In that way my Nginx Ingress controllers would get the correct X-Real-IP and X-Forwarded-For values.

Relevant Links:
HAProxy send-proxy docs: https://cbonte.github.io/haproxy-dconv/1.7/configuration.html#5.2-send-proxy

[ggreenway]

Duplicate: #1031