add SocketProtocol for DoT and DoH

[pspacek]

enum SocketProtocol probably needs new values for DoT and DoH. These two transports are becoming more prevalent but dnstap cannot capture them.

[cmikk]

I would like to discuss support for these transports in more depth on the dnstap mailing list before adding these values to the enum.

My main concern is the potential need for more information to make DOH instrumentation usable, given that DOH makes use of HTTP/2 mechanisms to match queries and responses.

[pspacek]

That's certainly possible, but I do not think it precludes adding new values to enum.

[pspacek]

@cmikk Hello! It seems the issue slipped through cracks. Do you want to start the discussion on the mailing list?

[cmikk]

@pspacek - I can start this discussion early next week, unless you want to kick it off sooner.

Thanks,
-Chris

[pspacek]

I would appreciate if you start, it seems you have better idea where the problem is. Thanks!

[vcunat]

Started: http://lists.redbarn.org/pipermail/dnstap/2020-December/000103.html

[edmonds]

I like the suggestion from multiple developers to simply extend the SocketProtocol enum rather than adding a new field to specify DoT, DoH, etc. My reasoning is here: http://lists.redbarn.org/pipermail/dnstap/2020-December/000108.html.

[cmikk]

Taking "no comments on PR" as agreement, and merging.