transport/http: ability to log response headers, response size, etc.

[peterbourgon]

With transport/http.ServerFinalizerFunc, it's possible to log a lot of information about the incoming request—especially when combined with transport/http.PopulateRequestContext.

Using the same mechanism, we can log the status code of the response. But it would be great to capture a lot more information about the response, at a minimum the response headers and the response body size.

/cc @xla