Exposing HTTP2 "additional debug data" in received GOAWAY frames about gun

[rnowak]

The third value in the goaway tuple, the error code, would also be great to have access to. This is already translated in cow_http2 by parse_error_code/1. I've so far seen 0 / no_error and 11 / enhance_your_calm emitted in the wild.

[essen]

If the error code is not no_error (equivalent to "normal") it probably should be reflected yes.

[manas-chaudhari]

+1 for this requirement.
I am also willing to take a stab at implementing if I can get some guidance on the approach.

[essen]

The close reason just has to be propagated from gun_http2 to gun. Currently gun_http2 returns the close command, we would need a new command {close, Reason, Extra} that can then be given to the gun_down message. Note that if the Reason is no_error, we should handle it by using normal like currently and not propagate the extra data.

There is additional difficulty because:

• the connection isn't closed as soon as we receive the GOAWAY frame, so we need to keep the reason/extra data in our state until then
• the server may send multiple GOAWAY frames during the shutdown; we should keep only the most recent reason/extra data

The last item has the caveat that servers may not send the extra data multiple times, but we'll cross that bridge when we get to it. For now we should just leave a comment in the code indicating that because we keep the last GOAWAY information only, that data may be lost if the server changes what it sends in subsequent GOAWAY frames.