Unsupported codec on server should fail with Unimplemented instead of attempting to use proto codec

[dfawley]

grpc-go/server.go

Lines 1433 to 1435 in f1437f7

	if codec == nil {
	return encoding.GetCodec(proto.Name)
	}

This would be a behavior change, even though it's also a bug fix, so it would need to be carefully considered first.

[easwars]

@dfawley : What if we make this change, but have an env var that people can set to get the old behavior? Or, we can consider the other approach, where we throw a log saying "oh, we didnt find the codec, we are going to be using proto for now, but this change".

[dfawley]

Yes, I think we could start by logging a warning and then have an environment variable to revert to the old behavior for a few releases.

[easwars]

Added a warning here: #6658