support for proxy protocol v2 and AWS ELB custom TLV type

[tjstansell]

We'd love to put fluentd behind an AWS PrivateLink endpoint, but still be able to see data about which endpoint the consumer is using (which we can then use to identify which consumer is talking to us). AWS ELB supports proxy protocol v2 and includes a custom TLV type that exposes the endpoint id on the consumer side.

Similar to the add_remote_addr option in fluentd for http, we'd love to see an accept_proxy option that enables support at the tcp level to use the proxy protocol information and expose the TLV values as well.

• https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#proxy-protocol
• https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt