Add shadow-traffic / traffic mirroring feature

### What should we add or change to make your life better?

Is there a plan to support request mirroring to a second endpoint?
Let's say the reverse-proxy receives a request on 8080. In addition to forwarding it to the configured endpoint, it would "duplicate" the request and send it to a second endpoint (most probably in a fire-and-forget manner).
Configuration could define a percentage of requests to mirror, request modification etc...

### Why is this important to you?

In a production system, when testing a new version, it is very useful to be able to get real PROD traffic in without impacting the production environment. Being able to shadow some traffic to another fleet of boxes running the new version helps us a lot when releasing impacting changes to high-throughput services (1M+ requests per second)