Add information about selected peer to response

[i]

It'd be useful to add an option view response headers. In addition, a header that specifies the host that handled the request would be useful information for debugging.

[prashantv]

I think yab will print all response headers by default:
https://github.com/yarpc/yab/blob/dev/main.go#L384

To get the host that handled the request, we'll need to extend our RPC libraries to propagate this information back via headers. I've been pushing for this feature for a while, but it requires every service in the stack to back-propagate this information for it to work.

[prashantv]

However, yab can add information about which peer it specifically selected, which may not be useful when it's a proxy, but is useful in peer-to-peer cases.