Add API to get RST_STREAM value from HTTP exceptions

[JamesNK]

Part of User Story: dotnet/core#5493

Background and Motivation

gRPC specifies that RST_STREAM error codes are mapped to gRPC error codes - https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md#errors

I believe that today the RST_STREAM value is only exposed in exception message descriptions. The only way to get this value is to parse exception messages, which is pretty awful for the usual reasons.

There should be an exception property with the RST_STREAM error code so it can be used programmatically.

Proposed API

Make this value publicly accessible:

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2ProtocolException.cs

Line 28 in faf91e0

internal Http2ProtocolErrorCode ProtocolError { get; }

I don't feel strongly that it needs to be an enum. public int ProtocolError { get; } would also be fine.

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[geoffkizer]

I agree that we should expose this value.

There's a similar issue with error codes from GOAWAY -- this is similar to RST_STREAM, except it affects the whole connection and kills all streams. As part of exposing the RST_STREAM error code, we should also consider exposing the GOAWAY error code if/when appropriate.

We should also try to do this in such a way that it works for HTTP3 as well, since there is a similar error code capability in HTTP3. Not sure to what extent the error codes are aligned, though...

[JamesNK]

HTTP/3 error codes are distinct https://quicwg.org/base-drafts/draft-ietf-quic-http.html#section-11.2.3

[geoffkizer]

HTTP/3 error codes are distinct

Of course they are :(

And they can be 62 bits long, too.

I guess this argues for having something like long ProtocolError, with values that are version specific. But that raises the issue of how you know which protocol version was actually used...

[JamesNK]

It looks like distinct ranges for each protocol.

Http2: <255
Http3: >256