dnsdist: Support the full Meta values in protobuf messages

[rgacogne]

• Program: dnsdist
• Issue type: Feature request

Short description

Currently dnsdist only supports exporting a single value for a given tag key, while our protobuf message format now allows several values. It also supports a numerical type in addition to the existing string one.

Usecase

Being able to set more than one value per tag key from the Lua APIs (regular and FFI).

Description

This is a limitation of the design of the tags associated to a DNS query, which predates the Meta field in the protobuf message. I think the best option would be to refactor the design of our internal tags to support the integer type as well as several values for a given key. We would have to do that in a way that remains compatible with the existing use of the internal tags, while making the full capabilities available to the Lua APIs. It will require a significant amount of work, and unfortunately won't be self-contained enough to be backported to 1.9.x.