Add %ORIGINAL_DST% as a header variable

[mandarjog]

Add %ORIGINAL_DST% as a header variable

Description:
When the original destination ip as resolved by the client is disregarded by the local proxy, it may still be relevant to an upstream.

This enables configuration like

request_headers_to_add:
- key: x-original-destination
  value: %ORIGINAL_DST%

For reference istio via directive: https://docs.google.com/document/d/1BGi18QnpqceEMzGAPWlzOTvlu65QRvbiWtyWK9jtddU/edit#heading=h.ouc0ih1n6vge

[mandarjog]

I would like to take a crack at this.

common/filter/listener/original_dst.cc## OriginalDst::onAccept recovers the original dest address and stores it using socket.setLocalAddress(addr, true)
This is available as request_info.downstreamLocalAddress()

Will this only be the original destination if the original_dst listener is used? That is the only place I see this being recovered.

[rshriram]

Scrolling through Envoy users, I stumbled upon this: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Host (thanks to @9len)