Better full duplex streaming responses handling

[kfswain]

What would you like to be added:
When using full duplex, we currently buffer the response if the response content-type is application/json and passthrough(stream out) when the response content-type is text/event-stream. This comment suggests allowing a flag to declare passthrough for the json path. Alternatively, since we do no modification of the body and only capture the response to capture usage metrics, we could do all buffering in a goroutine and immediately stream the response back in the main thread.
#469 (comment)