rec: Add bindings to set arbitrary key-value metadata in logged messages

[chbruyand]

Short description

This PR adds new protobuf meta field that can contain arbitrary key-value metadata.

This still needs:

• regression tests
• the corresponding non FFI interface ?

New protobuf meta field can also be read as the following protov3 map:

  message MetaValue {
    repeated string stringVal = 1;
    repeated int64 intVal = 2;
  }

  map<string, MetaValue> meta = 22;

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[omoerbeek]

Rest of review will follow once this is non-Draft.

[omoerbeek]

Looks good to me. A few nits and docs are missing. Also, it would be good if somebody more knowledgeable about ffi interfaces takes a look.

[rgacogne]

Looks good, I wonder if we should exclude lua-recursor4.cc from the formatting, at least partially.

[chbruyand]

I turned off the clang formating for the whole RecursorLua4::postPrepareContext() function and kept it on for the rest of the file. This looks indeed better like that.

[omoerbeek]

Does this need a backport? If so, the earliest version it's going to be in is 4.5.5, as 4.5.4 was just released without this. Otherweise the versionadded:: should be 4.6.0

[omoerbeek]

same here

[omoerbeek]

LGTM!